Its all in the pronunciation.
We learners of spanish have all come across the unpronounceable word. I have two that bedevil me, one for good reason the other for no good reason.
The one for me is "impermeabilización (tilde needed I'm pretty sure). I hack this word every time I try to say it, and I have to reduce it to syllables in my mind before saying it ( a real drag for the listener). The other (for no good reason) is "maderería), I can make a compete hash of that one as well, and it is not particularly difficult.
All of us have encountered these,either in studies or in daily life.
Post your "unpronounceable", and lets have the native speakers and our language experts help us with these words. It will be a good learning experience.
18 Answers
There is no good reason but the words "aun" y "aunque" never seem to sound right when I say them. It doesn't seem to be the diphthong "au" because I don't have problems with other words like "autobus", etc. Perhaps it is the addition of the letter "n" after the diphthong.
The word that really trips me up every time is estadounidense. I don't think I have successfully pronounced it yet. 
The tip for Silent Hero to use "nevera" for refrigerator reminded me of other words that I always confuse. I can say them and even know what they all mean when I stop to think about it. However, I often exchange one of these words for the other...especially the first two.
nueve
nueva/nuevo
nieva
nevera
Hopefully I will never be in a situation involving the need to discuss nine new refrigerators to keep snow in.
Necesito nueve nuevas neveras para la nieva nueva.
Paco,
For the sake of your sanity, I hope you do not have to use that word (Imperm.) too often!
I have found it an uphill battle memorizing a word I had to use in an oral presentation:
anglosajona
(I keep wanting to pronounce the "j" like an "x" from force of habit).
I have to admit, like renzo, I tend to try substituting shorter words!
For a native spanish speaker "theatre" is a challenge.
Nicole, let me give you a trick. Pronounce it: E S T A D O U - N I D E N S E. - usarenzo
I echo Paco's sentiments about the help you are providing. It seems that the number of people attempting to learn Spanish in this forum outnumber the native Spanish speakers. We have some very kind, patient and generous natives on this site. I am glad you are on board and I especially enjoy your threads. They are always a challenge. 
Thanks for the trick. I think it just might work for me. Fingers crossed! I am American, so I am expecting to need this word often. 
I can never say Seguro/Segura correctly. They always sounds really bad...
Paco, I figure you have to use it for your business, but
You know, Chivi was on to something!
But I would make it more extreme for memory's sake:
Imper Mea (like mea culpa) Billy (like boys name) zación
So:
IMPER MEA BILLY ZASIÓN
(let me know if that helps!)
I still can't quite say "refrigeradora".
The conditional also makes some long verbs hard to pronounce.
One of the ones that used to cause me to stumble and with which I struggled when saying it was not so much one word but speaking these words in a flowing sentence/question:
¿A qué te dedicas? = What do you do for a living? /What work do you do?
I can say it reasonably fluently now The only way I could master it was to break down the words and practise saying
dedicas then te + dedicas After managing to say that a number of times over a perido of about two or three months I then added qué + te dedicas then finally the whole question:
¿A qué te dedicas?
Anglosajona ¿por el idioma? Si ese es el caso, pudieras decir anglo o de habla inglesa
On the contrary, Anglo-Saxon (aka Old English) is the name of a language. Your suggestions apply to the speakers of modern English (or their descendants or, merely, their ethnicity) since Anglo-Saxon is a dead language.
I have also found out that I had been mispronouncing ...
Ah, you see what you have missed by not inviting me to anything but the infrequent intermediate/advanced chats. I would have commented on your pronunciation mercilessly.
In my experience it is so much easier to address the faults of an advanced speaker than of a beginner. Not only does the advanced speaker have the wider context to which to apply the correction (in most cases the advanced speaker can already produce the correct sound but does not realize that it is required in a given context).
Real beginners, on the other hand, are very hard to correct. One doesn't want to overwhelm them with corrections but when every other word is mispronounced, what is one to do? Where to begin? Which errors are more serious?
????????? After stumbling along in broken Russian for ten years now, this word still causes me grief. It means a 14-passenger van used in public transportation which follows a specified route and stops, but most of the time - unlike busses - you can get off and on almost anywhere. It became easier to pronounce after I was told to say it as two separate words: ???? - ????? (Marsh - rootka). The R is rolled in Russian as well.
We have this previous thread on this topic.
It´s always good to be reminded that some topics never end.
For me, I find I overcome most problems with a little work. I find the most difficulty in words that are very similar to their English equivalents, especially when they involve the silent ¨h¨and the soft ¨g¨ - for example: higiénico
I really only have trouble saying a word when I'm speaking Spanish quickly and I kind of jump over it. If I think about it and say it slowly it usually comes out pronounced correctly. I find Spanish words easier to pronounce than English, mostly.
Talking on skype with people I have found that easy words(I think they are easy) are actually not that easy at all.
pero often sounds like perro
I will ask Jenny or Marianne to answer here, they will remember.
I have also found out that I had been mispronouncing:
(My stress bolded)
centimetres
Video (long ee)
conjugation (j like in mayor)
So sometimes even people with some practice have aquired "vices", difficult to overcome
