Not checking headspace after changing annealing practices... was sizing to 0.004 shoulder bump. Within the next two firings all 100 lapua cases were case head separating and toast.
Most frustrating mistake is when loading precision rifle rounds on my Dillon 550 with a funnel to pour in weighed charges. Pour a charge when the ram is down.... powder everywhere. Double charge when the ram is up, then when you lower the ram.... powder everywhere. Needless to say I'm weighing the full cases after loading to confirm no empty cases/squib loads.
I'm only making that mistake every other loading session or so.