There are plenty of legitimate reasons to leave a job. You had differences of approach. You wanted to explore new opportunities. You stole a company truck, drove it to Mardi Gras, and ran a woman over. You know, the usual reasons...
This piece is based on a number of AskReddit threads. Links on the last page.
1. BIG pharma.
I was promoted to VP of my company. The company was in trouble, and the CEO had asked me to figure out why and fix it.
I arranged for a random drug test. All employees, the CEO, me, everyone. All on the same day and everyone went down at the same time. Even said I would ignore weed but anyone with anything stronger would be gone.
We get the results back and I fired everyone who had tested positive for any drug other than weed.
The CEO and myself were the only two people left working for the company.
2. Smoking gun.
Found a gun in the desk of a guy who we would frequently half-joke about (behind his back) being the one guy who would shoot up the office.
3. The chair man of the board.
Caught an underage restaurant employee stealing liquor and drinking it on the job. He got so drunk that he couldn't stand on his own and he was trying to put a chair in the dishwasher.