The person who coded the change to the mouse wheel in Firefox without restoring the original behaviour.
Most users will end up looking the fix up and applying it because the current set behavior is annoying. That's 5 minutes of the their lives they won't get back, now multiply this a few million times and it's quite a massive time sink.
The people who decided that <tt> is going to be obsolete in HTML5.
The Subversion website has 2559 <tt> tags and as many closing tags. In theory, a simple find-replace will suffice, in practice that's 5118 changes that can go wrong. Billions of hours will be wasted on this repair job all over the world :(
Time is the only true possession we have. Don't waste it.