amthomas wrote:

If you speak with a rather pronounced accent, and you try to use really complicated words like "quantum disentanglement" or something like that in conversation, people will assume that there is no way that you could have said "quantum disentanglement" because you can barely say "Hello my name is Joe" without making a mistake.

And that's different from the US how?  If someone comes up to me with bad grammar (leaving out 'the', 'a', etc) and says something of that complexity, my first reaction is going to be to try to figure out what they really said.  The next will be to have them repeat it.  If they really do manage to repeat it a couple more times, I'll accept it and continue on. 

I would do this even to people I know well.  It's just human nature to assume something/someone is only going to be on 1 level of complexity and not skip around from very basic to extremely complex.  When a computer interface does this, it confuses the user beyond hope of learning the interface.  When a human does this, it makes them very hard to understand.

When my niece at 3 years or 4 years old came up with the word 'abomination', we all just stared.  Then we said 'What?'.  So she repeated it:  "I would be an abomination!" she said.  She had been watching Lilo & Stitch.  She said it perfectly and there was no real doubt as to what she had said, we just didn't think it possible.

Probably the biggest two mistakes JSL people make is overusing pronouns/over-repeating the subject, and speaking too politely (and thus effeminately if they are men). Neither of these are grammatical though.

esgrove wrote:

We all know the common mistakes that a Japanese learner of English tends to make in their accent. Their R's and L's get confused, their "er" sounds like "aa", they say "shi" instead of "si", Their V's sound like B's, and they tack an extra vowel on to the end of words that end in consonants.

I was interested in the common mistakes that a native English speaker makes when speaking Japanese. According to one woman I asked, the biggest one is that we tend not to pronounce drawn out vowel noises.

For example　しょう　sounds like しょ, and　とお　sounds like とう and と

As I result I started self-consciously over pronouncing the last vowel, and now people can't even understand me. When I say to a bus driver 高校で降ります it sounds like こうううこうううでおります (although if I were to underpronounce it he thinks I want to get off "here", I guess you have to pronounce it exactly right)

Are there any other common mistakes?

Yeah I've also experienced this.  It seems pretty darn hard to get the small tsu and the long vowels down perfectly.  And when I'm nervous or not thinking I find myself saying schwa vowels a lot and not pure ones.  Really annoying how easily it slips in.  I recently asked my tutor to stop me for every mispronunciation and it's been pretty taihen.

One constant mistake (tip) I do (have) is to consistently accent the second syllable of your words. (obviously there are exceptions yadda yadda) A lot of times we will accent them according to how they fall in the English sentence or how we imagine the word being, and confuse the listener.  Especially with 2 syllable words I (we? anybody else?) accent the first syllable a LOT, and I've come across people that can't even understand the word if I say it like that.