At my previous company, although this wasn't the reason it's my previous company I hasten to add! I accidentally deleted an entire project the day it was due to be completed. Unfortunately I'd very stupidly not committed it to SVN.
I then compounded the mistake by covering up for 2 weeks trying to redo it franticly saying "sorry it's been delayed, issues in debugging!"
After 2 weeks I had to admit my mistake, I wasn't in trouble for the accident, I was in hella trouble for trying to cover it up!
Now I just tell people as soon as anything goes wrong on a project, even if it's my fault it's gone wrong. My current job is a SharePoint / Project Server consultant on Windows instead of a Ruby on Rails, Python and PHP in-house Dev on Unix, so disclosure is even more important so that it can be replanned with the client etc!