My boss is cheap so:

- My machine randomly crashing
- Internet disconnecting

On another note:

- Clients calling every 5 minutes instead of putting together a list and coming back when they have more than one point at a time

A customer forced you to use Comic sans and now you can't use it for your references..

Checking for a hardcoded password and submitting User and password in a hidden Input each time.

Didn't know about Sessions back then

The One thing every Dev should know about:
Never Push to production on Friday.

Coworker pushed to production yesterday.
Customer saw it and likes the new features.
Customer also saw a shitpile of Bugs.
Customer is angry and my weekend is runined