Two things actually bad I do :
* Put some printing lines everywhere to debug, and then, debug.

* A syntax than can be pretty bad if not handled properly :

if (your test)
do_something();

I actually always put a tabulation to see the hierarchy and break lines, which is not optimized AT ALL but help me to read, and I hope helps other to read too.
But that's a bad habit tho if you have bad presentation of your code (which I don't have, given how people compliments my code presentation)