maybe not really a bug but it can be nasty when you encounter it:

if someone edits a css file in an external editor and adds css classes below the /* EDITOR PROPERTIES - PLEASE DON'T DELETE THIS LINE TO AVOID DUPLICATE PROPERTIES */ comment, all this content gets deleted when you open this file in the umbraco css editor and save the file again.

this can result in loss of the added css rules.

I propose an alteration of the message warning users not to edit any part below this line, nor add additional css below it.