The guy literally fucked around in prod for an actual fucking client, changed configs without creating back ups of any kind, didn't communicate the downtime to end users, so we got shitton of tickets. It was Samba configs and fstab iirc.
He though nothing of it and since he was a "Senior", not even a slap on the hand for that shit. Oh and now we have to manually mount one of the shares after every reboot, because for whatever reason it won't mount automatically. Most likely fucked up kerberos config as well.
Same guy also suggested running script he got from the internet on prod without understanding what the script did. Good thing other guy took a look at it, because he'd overwrite our configs and would be pain in the ass to fix it.
We're currently running some ancient Nagios checks written by guy who's not working here anymore, some alerts we get are actually false positives 80% of the time, but current "monitoring expert", have no idea how to fix it and will not create new checks.
We had self proclaimed powershell expert in one of the previous jobs, but when I took over maintenance of one of the bigger scripts, he couldn't tell me literally anything useful, not for the lack of trying either.
We started new project and one of the guys who supposed to lead knowledge transfer can't answer a single question. "Ask <The Other Guy>" is his go to.
I have more, actually, but I'll leave those for some other time
