I have collected some "Don't"s in my lightning talk from
in reply to Little annoying mistakes ... of others
These are mistakes I made myself or found while refactoring
code from others, or when answering perl questions in forums
edit: It's a bit short, so my example why one shouldn't use C-style
for loops is missing the reason why I decided to include it - I found
an off-by-one error. And of course every rule has its exceptions.