Lotus Notes - the horror, the horror..
We've all had bad software experiences. However, at my last job, our corporate email client of choice was Lotus Notes. And until you've used Lotus Notes, you haven't truly experienced bad software. It is death by a thousand tiny annoyances -- the digital equivalent of being kicked in the groin upon arrival at work every day.
It's so bad that there are websites dedicated to documenting how much Lotus Notes Sucks:

Lotus Notes is a trainwreck of epic (enterprise?) proportions, but it's worth studying to learn what not to do when developing software.