@thegibson not my worst one, but the funniest one:

Fucked up the computers of a thousand+ of our lanparty guests, since we set DHCP option 46 to only use WINS and not broadcast for NetBIOS name resolution (p-node).

Windows set that flag permanently and people were not able to find other computers back at home anymore.

Solutions was of cause editing something in the registry by hand to restore the original behavior. Took us a week to figure that out, people were pretty pissed 😅