Well, there are honest mistakes (ie. the student honestly doesn't know or remember), but I think the biggest mistake is ignoring the teacher and assuming it's OK to speak sub-standard English. This includes broken sentences, lazy pronunciation and wrong tenses, as well as Su.Ki.'s examples of students using "wanna" and writing "u" and believing that it makes their English better. Other lovely comments from students have included... "I don't think I'll ever need to use that." (Oh yes you will.) "Do I really need to know that?" (Yes, you do.) "But in my language, we say..." (That's nice, but this is English.) "Is that really common to say?" (Common enough to make it worth you knowing.) "Oh, English is really hard." (...and complaining about it makes it harder, don't you know.) :)

This is really a valuable question to ask. I'm just answering here to keep this question alive :D we need more teachers to answer

Missing out the auxiliary 'do' from questions!

As a former Teacher, I'd say the use of tenses. Many students always use the simple present tense to talk about every experience.