Apart from the usual classroom management, discipline and grammatical mistakes, I suppose the hardest thing was getting the level right when speaking to the class e.g. talking to beginners too fast and using language they'd have no chance of understanding (phrasal verbs, present perfect) and making things too easy for the higher levels by talking more slowly than I should have.
As for gaffes made when using
their language, I have loads
Like the time I went to the bank and said I wanted to make a cheque really angry
, or worse, after lunch with my then new girlfriend's (now wife's) parents saying that after such a great lunch I was ready to make love!
(I meant to say I was ready to burst!)