The top ten mistakes
According to this website, many people commit the following mistakes in bold...
10.False: The word "día" is feminine. The truth: The word "día" is masculine .
9.False: Things in Spanish translate word for word into English. The truth: Do not translate word for word.
8.False: "Estar" is always used for talking about location. The truth: Use "ser" to tell where an event is taking place.
7.False: "Aquí" and "acá" both mean here, so they're interchangeable. The truth: The words "aquí" and "acá" are not interchangeable.
6.False: Vestido means vest, right?! The truth: Don't be tricked by false cognates.
5 False: I have to use subject pronouns with every sentence. The truth: Don't overuse "yo."
4 False: Only people from the USA are American. The truth: Spanish speaking people consider anyone from either North America or South America to be an "americano".
3.False: Only the future tense can describe the future. The truth: You can use the present tense for the near future.
2.False: Fecha and cita are interchangeable. The truth: Don't confuse "date" with "date."
1.False: Vez and hora and tiempo are interchangeable. The truth: Don't confuse "time" with "time" or "time."
So, do you disagree or agree with this list?
Also, which of the above are you guilty of? Come on, be honest! 
17 Answers
Another common mistake I have made is using haber + past participle to mean that I am continuing to do an action that began in the past.
Example:
1) He estudiado español durante dos años.
(This means that I no longer study Spanish because the action has ended)
2) Llevo dos años estudiando español.
(This is more accurate when I mean to say that I began studying two years ago and continue to study)
8.Use "ser" to tell where an event is taking place.
"Ser" is actually used to tell where an event is taking place:
La fiesta es en mi casa.
La reunión es en el segundo piso.
"Ser" is not used to tell where objects and people are located.
Carlos está en mi casa.
La oficina está en el segundo piso.
3.Use the present tense for the near future.
Present tense (not progressive) is actually used for any future, all the time:
Mañana voy al circo.
El año que viene voy a Jamaica.
Dentro de diez años me jubilo.
However, the present progressive is not used for future events, but for actions that have already started:
Estoy viendo el circo. [right now - it has already started; it cannot be in the future]
Estoy cantando una canción. [right now - it has already started; it cannot be in the future]
2.Don't confuse "date" with "date."
Clarification: do not confuse the date that gives day, month and year (fecha), with a "romantic" appointment (cita).
1.Don't confuse "time" with "time" or "time."
This one is harder to explain in just a few words. Check previous posts.
Choosing the wrong mood has to be on there somewhere.
I don't agree with 2. I've never seen anyone do that.
Plus, mixing up preterite/imperfect isn't on the list. That's a very common mistake. It's one I'm guilty of.
Two extremely common mistakes (among beginners):
1) To translate the English gerund as a Spanish gerundio (most of the time it is an infinitive, and the rest, you need to rewrite the entire sentence):
Dancing is fun -> Bailando es divertido 
[it should be "bailar"]
Gracias por ayudando -> Thanks for helping [it should be "por ayudar"]
However, the present participle, which looks exactly like a gerund, is very often translated as a "gerundio":
Estoy pensando = I am thinking
2) To translate any "to" as "a" (this 'trick' fails far too often), especially to mark the infinitive of a verb and to indicate resulting condition:
I want to eat -> Quiero a comer [it should be "Quiero comer"]
I am glad to help -> Me alegro a ayudar [it should be "Me alegro de ayudar"]
The preposition "to", however, can be translated as "a" when it indicates direction or end point ("going to the cinema"), and to indicate human grammatical objects ("Give it to him").
3) To say "Gracias para + [verb]" (meaning "thanks towards [verb]"). Why do all beginners always chose "para" the first time? That's still a mystery to me. "Por" is used statistically almost twice as often as "para".
Aca and aquí in theory aren´t, but in some places acá is preferred. The only time aca is not interchangable is when you´re talking about degrees. Actually you can use the present indicative to refer to planned future actions and you will sound m You would not use the present progressive to denote future actions like you would in English. Hablamos pronto could meanswe´ll speak soon.
I have certainly been guilty of committing the first 3 errors in the past no's 10 ,9 and 8 and while I have also used personal pronouns too often I have never considered it obligatory to use them in every sentence . It is amazing how being sent to the dunce corner a few times by Heidi can cure you of overuse of personal pronouns hehehhe 
I have been learning from my mistakes and am confident that I will not be making many of them again 
What about "para" vs "por"? I've completed two years of Spanish in school, and honestly, I still have no idea when to use which one. I'm sure it's not just me.
I have undoubtedly done most if not all of these but have gotten over most. 8 and 3 I wasn't aware of so I'll have to work on those. Great thread, Sonrisa.
I could've sworn "cansado" [tired] had an accent...
Huh? Where would you put the accent? The stressed syllable is the penult! For words ending in vowels this is the default position. Aside from the accent marks that signify a grammatical distinction (e.g. "como" / "cómo"), the use of accents in Spanish is covered by a very few simple rules (assuming that you know how to correctly pronounce the word). Would that English orthography were so easy!
When people say "No problemo", drives me crazy!
4.Misuse of the term "American."
This one can really get up the noses of people living south of the Mexican - USA border.
I've given día a sex change a couple times... 
the list seems pretty accurate to me. I might give or take one or two, but hey, it's a general list, and they probably study a more broad group than I.
Another mistake is putting the accent in the wrong place, adding one that is unnecessary, or omitting it entirely. I do that sometimes, too.
I could've sworn "cansado" [tired] had an accent...
A quote from Garry Marshall, 'Wake Me When It's Funny'...
"It's always helpful to learn from your mistakes because then your mistakes seem worthwhile."
So true, everyone!
I personally feel that mistakes are the only way we can truly learn.
