I created a blog that talks about 3 quirks of maps/multisets.
- Accessing non existent elements in maps keyed on characters may lead to WA.
- Accessing non existent elements in maps keyed on integers may lead to TLE.
- Using the fancy count operator in multisets may lead to TLE.
There's no new/unique insights, I just documented my personal experience with these quirks, along with the problems link where I first encountered this issue. It might be helpful to beginners in competitive programming.
https://cfstep.com/training/tutorials/general-techniques/quirks-of-non-existent-map-elements/