It seems like certain games and systems always have the same problems or parts that are missing. Here's my list, I'm sure the gang here can list many, many more.
VCS Activision carts- actiplaque, it goes without saying
VCS Sesame Street carts - actiplaque
VCS Parker Brothers carts - peeling labels. At some point they must have switched to a cheap glue; Frogger and ESB carts are usually fine, but I don't think I've ever seen a Gyruss or Popeye cart without a peeling label. I've seen more Parker Brothers games without labels than I care to count.
VCS Konami carts - the labels for these were all one piece. Always seem to be messed up.
VCS paddles - nuff said
Odyssey 2 big box games - Quest for the Rings, Conquest of the World, etc. are always missing pieces
5200 console - nuff said
7800 console - often missing the power adapter
Vectrex 3D imager - there is a foam piece that goes over the bridge of your nose, that piece always falls off
Coleco tabletops - OK, all tabletops - missing battery cover. What did kids do with these, eat them?
Coleco tabletops - actiplaque and peeling labels
NES systems - blink
NES Arkanoid controllers- usually broken
SNES systems - yellow, cover on cartridge slot seems to be missing a lot
32X - either can't get it to work, can't get it to stay working, or games crash
3DO FZ1 systems - I've had 3, one kinda works
Playstation long box games - lots of peeling labels, broken plastic on the piece that holds the CD in place
Playstation games - getting hard to find complete games. I'm rooting for the PS3 to not be backwards compatible so some of these will get coughed up.
Saturn system - dead battery, can't save
Saturn games - cracked cases
Dreamcast games - in my experience, the most non-working games of any system