I have so many. Here are a few. I've started with the most spectacular, physically speaking. Please enjoy.
Parent (male) tried to land their helicopter on one of the sports fields during a match ... You can imagine the terror this caused. (Parent subsequently banned from attending all school matches no matter what mode of transport he chose to use).
Parent (male) threw a wad of cash at one of the receptionists with the words "Take that, bitch" after being repeatedly chased for late payment of fees. (We didn't accept school fee payments in cash - everything must go through a bank in order to comply with money laundering regulations/obligations). Watching him pick all the cash back up was hilarious, especially as it was a really windy day and pupils kept innocently walking in through the big automatic doors as he'd foolishly decided to come at break time, so the wind was blowing all the notes around everywhere.
Parent (yes, another male) uttered the immortal words to their child during a tour of the school, whilst being shown a lesson in progress: "Don't worry XXX, if everything goes horribly wrong you can always become a teacher like this poor lady here." This poor lady was an ex-City finance whizz who'd done a late career change into teaching, so she was used to dealing with entitled males, and she actually came over to where we were standing and put the idiot in his place in style, super-politely but very deftly . She was absolutely amazing.
Some of the worst parent behaviour doesn't consist of one big, awful thing but rather a series of "small" things that build up over time. So - parents who talk negatively about their children in front of them (and, conversely, those who praise their children far too much and in situations where it's totally inappropriate). Boasting, and equating academic success (and/or money) with success in general. WHAT ABOUT HAPPINESS??!! Looking down on people with less money / fewer assets. Refusing to believe that there has been an element of luck in their "success" (yes, being sent to a private school / working in the family firm / inheriting loads of money IS lucky in most cases, even if parents claim that this is purely due to their hard work and not due to any luck at all).
Parents who aren't polite to school staff are a huge bugbear. I think I mentioned in a previous response that sometimes things get emotional when dealing with an unhappy child, and obviously that is understandable. But parents who are rude for the sake of being rude are just dreadful. I don't know if they know that they are being rude, or if they do know and they simply don't care. Probably some of each. Just general tone of voice can be so aggressive. Some parents just seem to be incapable of sounding polite at all.
Another bugbear is parents who do not read emails properly. Argh. Yes we are all busy, but that is no excuse. If I send you an email entitled "Entrance Exam Arrangements for Tuesday 24 March 2026" and the first line of the email says, in bold and underlined, Entrance Exam Arrangements for Tuesday 24 March 2026, PLEASE DO NOT reply with "what date is the entrance exam"?
Also parents who falsely claim not to have received a particular email, thereby wasting everyone's time in re-sending it and going through the same details over and over on the phone. It's always the email that contains information they don't like - funny how that always happens! (The system at our school was very detailed and we could see exactly when an email was received and opened).