I've been an ESL teacher for a few years now and I've heard some pretty funny things. One of my favorites is the German tendency to confuse wife with woman so you end up with sentences like: me and my woman went to the football (mind you I still confuse the words humid and gay in German to everyone's amusement, so I'm not one to talk). Japanese and Korean students had many other errors which were odd, or interesting, or bizarrely sexual. However, the best mistake I've heard yet was made by my 18 year old elementray student today.
The background is this: we were doing an activity where we had to guess what each other does at particular times of the day. As in: at two o'clock you work, at ten o'clock you sleep etc. In German the words for eat and food are the same, so you say I cook eat. The word for cook is kochen, pronounced with a soft ch in the back of the throat. So the activity is going very well and we're almost finished when my student has to guess what I do at seven in the evening.
My student: you eat c*ck at seven.
Me: (best poker face you ever saw) Nooooo... I think you mean cook?
Him: Yes, c*ck.
Me: COOK. C-OO-K. I cook my food for dinner.
Him: You c*ck your dinner.
And so it went on. I'm looking forward to the other teacher tomorrow being asked about her nighttime activites. If his English wasn't so bad I'd almost suspect him of knowing what he was saying.