As a former TEFL teacher, I entirely agree, Sarah. In my experience, points 2 and 3 here tend to combine into the unwanted result of 'I've learned my idiom, I love my idiom, and I'm damned well going to make sure I use my idiom and keep it it nice and shiny, even if it's really not appropriate in this context'.

Further exacerbated in many cases by slightly awkward stress and intonation, especially if learned from a book/website, which can make it hard to follow anyway.

It's a can of worms...