Mine is maggots.
It started years ago. We had just moved in to a house one summer.
We had to put our freezer in the cellar. It was also where all the empty boxes and junk went.
Everytime I went to the cellar, I kept getting a whiff of something, quick look, can’t see anything.
Went down to cellar for tidy up, just removed a box off another box, and found a pile of writhing, pulsating maggots.
I think that’s one of the few times I have really screamed. I’m fine with spiders and other creepy crawlies, live and dead mice.
It turned out a small nearly empty bag of frozen mince had accidentally been left out of the freezer. Eggs had been laid and maggots had been dining on 5* inclusive.
I later found out that maggots will crawl out of your dustbin, but the birds absolutely love them.