In 1997 I was working on an integration between AOL and Circuit City (ha I outlived them both) to enable free AOL accounts for people buying PCs or some such; about a week before launch I changed the data returned from encoding spaces as "+" to "%20" and broke their integration (perl script). Very upsetting for them, and I felt bad.

I also had some weird bug when we started registrations from German accounts and I didn't handle umlauts (or UTF-16 with nuls in the string) in passwords properly.