There are random times where osu! doesn't load all of my beatmaps on startup and only loads half of them. If this happens, any change to your collection will completely override the collections file to not include any of the unloaded maps. If you don't reload all your maps and restart osu!, your collections can get screwed.
Edit: Fortunately I had a backup when I went to the collections file Properties>Previous Versions, but it's still an annoying bug that could really confuse some players.
Edit: Fortunately I had a backup when I went to the collections file Properties>Previous Versions, but it's still an annoying bug that could really confuse some players.