Can't believe it hasn't been mentioned, but "minute of deer", "minute of gopher" and any of the other variations. Bugs the crap out of me. That was a pretty original term when I read it in a Gun Digest or some such book about 30 years ago. Kind of appealed to me then. Fast forward to the internet, and my involvement on it probably only about 10 or 11 years ago, and I've probably read it about 5000 times. Or so it feels. Stop. Just stop.
Say-ko. It's Finnish. Not a Japanese watch. Sako is pronounced Socko. I can't believe how many knowledgeable guys say it wrong.
Lee-a-pold. It's loo-pold. Props to the previous poster who noted the correct pronunciation of Leupold.
.45 Long Colt. I understand that it was used to differentiate the various .45 caliber chamberings at the time. It even shows up on ammunition boxes, but it is still wrong. Just like the clip labeling.
Combat accuracy. I see that every time I have a judgment lapse and pick up one of those stupid gun fighting rags. I don't care how crappy a shot you are, or aren't, I don't consider doubling the size of your group through a mismatch of ammo, or poor mechanical accuracy, good accuracy, "combat" or otherwise! Get out here with that crap!
Pattern. "How does that pistol pattern?" Probably alright if it was a shotgun. But it's not. It has a rifled barrel.
Tacti-cool.
Fan-boy.
Wanted to trade. "No junk, no Savages." Shut up, newb. You wouldn't know the difference between brands if you got butt-stroked by it.
ARRRGH!