Is there something wrong with me?
Since I have been learning Spanish, whenever I am in the vicinity of a Spanish speaking person or family, whether in the store, around a pool on vacation, etc., I always find myself listening in. The reason I do this is to see how much of their conversation I can understand.
In the beginning, I could only pick a word out here and there. Now I can understand phrases and sentences. I think it is time for me to stop this habit, but it is so tempting because I use it to measure my progress. The trouble is, now I feel like an eavesdropper. Is there something wrong with me or does anyone else do this? 
18 Answers
I've never been a superhero before, but (this is related to your question) I think I understand them. You see, you and I and all the other Spanish learners have to deal with an issue that superheroes also must deal with. 
Now, this may be an over-simplification, but when you really think about it, aren't superpowers nothing more than just abilities that most people don't have? Of course we all know that Superman was just a normal Joe on planet Krypton. But among Earthlings, he was, well... Superman.
So basically Nicole you are developing a superpower: The ability to communicate in Spanish. And that is an ability that most gringos in your area don't have. Now, to be honest, if you were in Hispanic Land you would be just a normal Jane (or María?), like Clark Kent on Krypton. So it's not a 'planet-Earth-wide' superpower, its more of a localized superpower, like in your town or neighborhood. However it is, using the criteria stated previously, a superpower nonetheless.
The issue you are currently grappling with is: It's new to you. Now you are faced with learning to control your superpower. I find it hard to believe that Superman didn't accidentally knock down at least one wall by walking through it, not realizing he had actually been 'seeing through' the wall where there was no doorway.
What if you could see through walls? An honest person would make the conscious decision not to do so unless it was for the greater good. Of course an arch-nemesis type person would take advantage of such powers to build a massive global empire. Can you see through walls? Well, when someone within earshot starts speaking in Spanish, it is no longer babble, but intelligent communication. There's no longer a 'wall' blocking those words. You can 'see' right through it. You understand!
So what are we to do, us superpower-developers? Well, I know that I don't like other people eavesdropping on me. That's one factor to consider. But on the other hand, people really shouldn't be talking about things in public that they don't want others to hear. That's another factor. But I think the biggest factor about this particular superpower is that the Hispanic people around us probably wouldn't expect us to understand Spanish and are therefore more likely to speak about whatever they want assuming that no one around can understand. This could lead to some unavoidably embarrassing situations that must be controlled. (Like the time a waiter said something in Spanish to another waiter about a physical trait of mine, obviously thinking I didn't understand. I did.) Do we use such situations for our 'global-empire' plans? Or do think of the greater good? That's a sobering ethical question that must be answered by each of us personally.
So the bottom line, as far as I'm concerned, is 'No, Nicole, there is nothing wrong with you.' However...
use your powers carefully... 
My wife and I don't look like Hispanic (if we use the American definition of Hispanic). I don't mean any disrespect saying this. Is just how other people see us, and we don't feel we are better or worse than people who fit the American definition for Hispanic.
Coming back to the point, more than once, we use this to have fun. Walking in into a Mexican restaurant we have heard funny things several times.
Once, my wife was watching TV in one of these restaurants. They were showing "el chavo del ocho", a Mexican show, and she was laughing.
A waitress, looking at her, said in Spanish to a coworker "look at this stupid American woman, she is laughing and can't even understand a word". My wife looked at her and replied "para tu información, soy de Argentina, y entiendo perfectamente". Poor waitress didn't know what to do. The colors were all over his face.
We've had a lot of these situations, and fortunately they have always ended in the funny way.
I don't think this is better or worse that what you do. Don't feel bad. We don't, and we certainly enjoy it.
Nicole, I think there would be something wrong with you if you didn't do that. I mean, come on, you're investing all this time and energy, learning something you love, with one of your heart's greatest desires being that of learning Spanish to the point of being able to converse, and yet you spend all of your time living amongst gringos. It must be maddening! 
Just like a stay-at-home mom longs for some adult conversation, you long for it en español. That's the way it should be!
If you were using a hidden microphone, that would be a different matter (that violates the "reasonable expectation of privacy". However, if they are carrying on a normal conversation in a public place, then they are "fair game". The fact that they may assume that you don't understand Spanish (based on your features, clothes, whatever) is their fault not yours. Had they been speaking in English would you feel guilty about not being deaf?
If there was a Spanish speaker within 200 miles of me, I'm fairly sure I would seek him out just in order to eavesdrop!
However there isn't one, as far as I know, so I am spared the temptation.
Sadly I don't much opportunity to eavesdrop on Spanish-speakers - there are so very few where I live. However - I did once...on a train, a group of women travelling to a holiday in the Blue Mountains for their once-a-year-gettogether. I eventually went up, told them I was learning Spanish and ended up sitting with them for the rest of the journey. They were just lovely.. Would I do it again? (eavesdrop that is) - absolutely!
I work at a restaurant and I do this all the time. English people tend to yell out whatever they are doing as if they don't have a care in the world "Oh yeah!! My dog?? yeah!! she died!!" etc.. but most of the Spanish speakers seem to trying to have a private conversation and I will find myself inching closer to their table.....little closer now, don't mind me I'm writing in my waitbook....little closer now......lol. 
Unfortunately, the only opportunity for me to listen in is to listen to some Spanish podcasts... 
Is there something wrong with me or does anyone else do this?
I do this every chance I get! ¡En toda ocasión, siempre!
Don't regret it, Join The Dark Side!
It's good practice, just don't do it to anyone who looks like they are in the mob, you may find yourself with cement shoes if they thought you overheard them talking about any assassination plots...
I see no reason not to listen in, I know I do it whenever the opportunity arises, I'm just not obvious about it.
...it is so tempting because I use it to measure my progress. The trouble is, now I feel like an eavesdropper. Is there something wrong with me or does anyone else do this?
Guilty as charged 
Oh my goodness...my girlfriend and I JUST talked about this very thing after our lunch date today. I'm an African-American female and she's an Hispanic-American female (who doesn't speak Spanish). Her husband (who's fluent in Spanish) and I encouraged her to sign-up with me a couple of years ago to attend a once/week Spanish class, but she declined...stating that she didn't have the time.
NOW, she's constantly wanting me to eavesdrop on Spanish-speaking conversations...then tell her what they're saying! How ironic is That?!!!
I teased with her, just today, that I'm going to start charging her a fee for every time I do that (or at the very least, start making her pick up the tab for lunch)! I told her I'm starting to feel like Secret Squirrel 
Honestly, though, I do it even when she's Not around!
But I think the biggest factor about this particular superpower is that the Hispanic people around us probably wouldn't expect us to understand Spanish and are therefore more likely to speak about whatever they want assuming that no one around can understand. This could lead to some unavoidably embarrassing situations that must be controlled.
Chaparrito makes my point more clear. These people are standing within my earshot in public places. I don't lean in closer or put a glass up to a wall!! But as Chaparrito mentions, many might assume that they are safe. I really don't think anyone is safe with Spanish in my area anymore. At least every third or fourth person probably speaks Spanish.
Anyway, I think it is making up for the years when I worked with Mexicans who were constantly joking in Spanish. I was always certain they were making a joke at my expense or saying something inappropriate. When I told them this, they would laugh even harder.
I guess the lesson for everyone is...don't assume!
Now, if only I could understand what the Chinese women were saying when they do my nails!!!
I do it all the time, and have heard things that I didn't want to
Depends on how you're eavesdropping. Do they see you and have the "wtf" face when they notice you? Its still bad to eavesdrop. (Not that we all do it, because I certainly do sometimes.) Instead of eavesdropping, join the conversation, so you don't look like a creep!
