missed_shot: that happens to me once in a great while, whenever I use the priming mechanism on either my single stage RockCrusher or on my Dillon 550. It sometimes happens if you are seating small primers using a large primer cup and the primer isn't centered - the lip of the cup gets caught by the base of the shell holder and flips the primer upside down. I'm sure glad I check all my primed cases before charging them.
I avoid this problem using a manual Lee priming tool - that was I know all my cases are properly primed..... HTH. Mikey.