Worst thing you've seen another dev do? So many things. Here is another...

A developer purposely forged international shipping documents by 'hard-coding' data to get around international shipping restrictions (ex. we can't sell 'Widgets' to Germany...so under the category he would replace the value with 'light bulb'). He was 'under pressure' to make keep the money rolling in no matter what.

We were eventually 'caught', fined over $300,000 (which was better than the $10,000 per offense and we had thousands of offenses).

For this major frack-up, 'Rob' was promoted to manager of the International department, got to travel (including his wife) to several European countries, and eventually obtained a company-paid MBA degree.

'Rob' liked to joke about how he would sometimes have to pinch himself how lucky he was by working for such stupid people (yes, he used the word 'stupid') and how gullible government investigators were.
"All I had to do was say 'its a bug in Windows' or some other kind of nonsense and they believed me."

'Rob' quit 3 months after receiving his MBA degree (again, 100% company paid) and the international department closed due to some potential illegal activity.