missing an exaust stud?!?! wtf...

[spider_slayer]

well, i have just started to take my head off to install a metal head gasket (thats right, after 6 months i have just now started). while removing the exaust manifold i notice that i'm missing an exaust stud. my question is how the hell could this happen? this is the first time in a year and a half that the turbo has come off the car so i have just noticed it and most likely have been driving around missing an exaust stud and nut.

so, i am gonna order some arp studs next week and replace all of them. i would like to know if anyone has heard of anything like this. also whats a good way to tell if i need to retap the thread or not?