I hope this question is not too open-ended, but please don't hesitate to suggest edits if it is...
Mainly inspired by this question on what seems to be the misuse of a standard idiom by a Japanese employee, I was reminded of being told about バイト敬語 ("manual keigo") in the past: a slightly dumbed-down sonkeigo crash course handed to new kombini recruits, that covers the basics but often results in widespread misuse of proper keigo forms at the hands of young Japanese unused to this level of speech.
I know there were a couple very specific examples of classic mistakes (generally: confusions between sonkeigo and kenjôgo). Wikipedia (en) lists a few disputed forms (「おつぎのお客様」,「〜になります」...) present in the manuals themselves...
I'd be interested to hear if anybody has other examples of egregious misuse and why they are not correct...
Update: In retrospect, this was probably too open a question and a better fit for a Community Wiki (thanks @troyen and @YOU for helping me set this up!)