At my last gig I was working at a small ISP and my boss was asking why our throughput went to shit every time he checked the router web ui. I told him it was because the web server on the router uses up a lot of CPU time, and that meant the router couldn't process as many packets since it uses that same CPU for well routing, nat, firewall rules etc...so it's probably best to use the CLI instead. Boss says, "YOU DON'T TELL ME WHAT TO DO!!!" And continues to destroy throughput by looking at the web ui.

TL;DR Boss yelled at me for telling him how things work. Yay.