I now only do a teeny bit of supply.
Prior to retirement, I was a Head of Dept and then Faculty Head in Scotland. (Principal Teacher Curriculum.)
One mother was always complaining. A real horror.
I'm still angry about this: SLT forced me to change my seating plan because the daughter complained that there was a "smell" coming from the girl next to her. There wasn't. The girl was of a different race and the mother's daughter was simply a bullying little racist.
We always had complaints from that woman.
My the time the girl was a senior, she was in someone else's class, but the teacher was off sick so I covered. Next thing, I was subjected to a formal investigation for "bullying".
I was interviewed; every single child in the class was interviewed. The conclusion of the investigation was that I had done nothing wrong. Apparently, my bullying consisted of "Be quiet. People are trying to work." (I was told that I was exonerated because I had also told someone else to be quiet!)
The mother refused to accept the findings. "My daughter always tells the truth."
My response to my line manager was that, in that case, perhaps he should let the mother know that the daughter had informed the entire class: "My mum's a slut! She got [euphemism for impregnated] wi me when she was 15! That's why she put me on the implant at 15!"
Sadly, my line manager declined.