Back in my museum days, one girl I had supervised was promoted to a position equal to mine, but seemed to think she was now my boss (she didn't particularly like me). As time went by, she was making more and more careless errors I was having to catch and deal with, but when brought up (in meetings or with her directly), she would blame me or others for those errors (while also being sure to bring up anything I did wrong or she'd thought I'd done wrong). After awhile, she started making a TON of mistakes, in all aspects of her job.
It hit the fan when within the space of a few days: the accountant discovered she hadn't made any deposits in over a month (part of her job was to drop off deposits regularly from tickets, gift shop sales, etc); field trips/teachers were complaining that she wasn't returning calls; and one day she booked a large group tour with literally only me in to cover it.
When I walked in that morning, saw the schedule and flipped out, her first reaction was to blame me for "not noticing sooner" (scheduling was squarely her job, and I would delegate tours and other tasks based on her schedule- kinda backasswards I know, but that was the set-up at the time). Other colleagues reamed her out for the scheduling incident, and for trying to avoid responsibility for it.
I'd had it at that point, and finally wrote a long email to our Exec Director discussing ALL the errors I and others had to cover for, unaware at the time that the accountant was also complaining to the ED about the lack of deposits (no, she wasn't stealing them- the deposits were sitting on her desk and she just hadn't been doing it).
It also turned out that this girl was smearing me to the ED, so these incidents prompted further investigation which thankfully cleared things up on my end.
Aaaanyway, that basically lead to an "intervention" with the ED and some others from management sitting this girl down and reading her the riot act, where she claimed job burnout for the errors (she was like 23 years old at this point, though we worked in an admittedly stressful environment).
She was put on some sort of super-probation, her work was double-checked, the scheduling procedure was re-jigged, and she suddenly acted a whole lot nicer to me.