Don't shoot handloads assembled by someone else.
This ^^^^ pretty much covers it.
I would add don't shoot ammo obtained from someone else, either. When I purchased my Marlin 375 (.375 Win), it came with a couple boxes of 'Winchester factory ammo'. Not suspecting anything, I fired a few. They seemed safe enough but accuracy was horrible and velocity was very inconsistent. Couldn't decide if it was really bad factory ammo or bad handloads. Either way, I stopped shooting them and tore the rest apart. Nowadays I don't even consider shooting ammo unless I know for sure it is factory fresh or I've loaded it myself. The one exception is my hunting buddy's handloads for his 7mm RM - I've taken a couple shots using his 7mm RM at the range but only after watching him shoot from the same batch. His load is one I used for 20+ years and he has used since '98 or '99 when he got his 7mm RM.
