Went paintballing yesterday for the first time in about 3 years. My Gun hasn't been used since then.
I have a Tippmann 98 Custom (like most people) But i bought it off a friend of mine way back and it had a few modifications.

Response trigger (Excess gas pushes a piston forward which resets the trigger - essentially allows for full auto)
Expansion Chamber - Allows the Liquid CO2 to expand into a gas before entering the gas chamber. Allows for tighter ball grouping.
Bolt cover - so leaves and grime dont clog up the inner workings.

Anyway since i havent used it in a WHILE, i completely forgot that there was a huge problem with the firing system - until i got there and chrono'd in and my gun went spastic and chopped about 8 balls in 1 second.. needless to say i couldnt use my gun... well i found out why:

http://www.facebook.com/album.php?aid=2042099&id=37403282&l=aaf3a62237

The rear bolt and sear (thing that catches the rear bolt and holds it in position until you pull the trigger) were chipped and rounded off. the combination of both those being so crappy made it almost impossible for the gun to function reliably.

Long story short, i put some detail pictures up and i spent about $42 in replacement parts - a whole new back end bolt-system.

Hopefully this is the only thing wrong and i can finally have a working gun again!