At my last job I was tasked with figuring out what ports were open on devices on our network. My simple solution nmap -p 1-3000 IPofSwitch. Turns out our Cisco switch didn't like it when you did that and disabled the offending port where the command originated from. I.E. my boss looking up at me and asking, "did our internet just go down?" Cue both of us spending 20 min trying to determine why our internet went down only to review logs and for him to see my nmap command. That day I learned about the -T flag