I wish I could learn to crimp cards. The only problem is that all my decks are the 100% Plastic ones, which means the crimp becomes permanent till I break the card off. And that's one tear no magician can restore.
However, my biggest regrets have not really been "not learning how to do'' a particular sleight. It has always been ''not learning how to do a particular sleight well".
There's nothing sadder than designing an entire effect based on sleights you ''know'' only to realise you didn't learn them well before, and now you need six months of practice before performing that effect.