Some "it's too stupid to check" issues I've experienced:
- Someone plugged a loose ethernet cable back into one of the switches, but the other end of said cable was already connected to the network
- Someone decided to make their own WiFi (with hookers and blackjack), but it started handing out new IPs to everyone
- Someone visited the switch room, and turned off the lights when they left, despite huge warning signs
- Logging all traffic caused router to overheat after 2-3 days