Dumbest I've ever heard of was a news article this week about some Russian nuke scientists that got arrested for mining bitcoin on a petaflop supercomputer that was supposed to be airgapped.

The dumbest user error I've ever personally experienced was on the side of the IT department. We had a consultant come in to "test the failover functionality" and "perform firmware updates" on our firewall system. What he ended up doing was unplugging the primary firewall and forgetting to plug it back in when he was done while scheduling the reboot on the secondary firewall for 11 PM on a Sunday night. I was paged into the office at 11:15 when nobody from the Shanghai office could VPN into the headquarters. It was great when I could unmute the bridge and say "the firewall is unplugged." I got the following Monday off!