Some mistakes I've made loading for handguns.

Too long of a COAL for reliable function in my autoloading pistol. I had an odd shaped bullet and got nervous about the seating depth. If I'd followed the book COAL in the data, I'd have been fine.

Buckling the case with too much roll crimp. Familiarity with use of the tools and help from forum users like on here helped correct it.

Too long COAL for the revolver chambers. Greater care and attention to detail cured that.

Occasionally I've had to deal with brass that was sub par or out of spec. Attention to detail again.

All and all. I think I've enjoyed my handguns more and enjoyed learning to load for them a lot more than I ever anticipated at the beginning.