I work in a primary school office. I try very hard to be polite and friendly to all parents and I find the school as a whole to be incredibly caring, much more so than I expected TBH, but there are some parents who make it incredible difficult to keep smiling!
There one woman, who has 2 DD in the school. Weds last week one of them had a headache. She told the teacher, but didn't really seem unwell, so the teacher told her to see how she was in a bit. When mum collected her, she was apparently terribly ill and mum was in the office next morning ranting about how DD had been left to "suffer" all day 
Next day the other DD had a trip in the playground. No visible damage, but lots of complaining about being in agony. After the girl's 3rd trip to the office I decided I'd call mum and see what she wanted to do. No answer on home phone (where I expected mum to be home with other sick DD!) so I left a message on mobile just saying that I couldn't see anything much wrong but that she was complaining of being in a lot of pain, did mum want to come and have a look? Mum took nearly 2 hours to call back and then decided not to come anyway. I wonder what she'd have done if I'd called about the headache?! Next day she paid both girl's dinner money entirely in coppers! 