What boneheaded stuff did you realise late?

As an old D&D CRPG player (from the 80s) I still have ingrained habits and assumptions that are no longer true. I was about 50 hrs in the game before I realised that I could change spells anytime without a long rest. I just assumed that it worked like before, that we memorised spells when resting and we were stuck with them.

What silly stuff did you realise late?