I made a doozy recently on a flight from BLI to LAS.
I'd been up since 3 am in order to catch a 5 am shuttle from Vancouver to Bellingham so I'm already running on a sleep deficit; when I get to BLI it's such a small airport that there are no signs telling you where to go/what to do, so I get in line with everyone else like a good little boy.
After a couple of minutes I notice that there's a second line which is moving a lot faster; I ask what the line's for and am told that it'd for those who've already checked in. "Fantastic," I think, "I've already checked in online so I can just switch over to that line and get through faster!"
Even after going past the TSA document checker, I didn't clue in to what I'd done wrong until I realized that there was no way in Hades that my suitcase, which needed to be checked in, wasn't going to fit through the x-ray machine. Yep, when they said "check-in line" it was referring to luggage too.
Fortunately they were nice and didn't hassle me as I pulled everything together and got out of the security line - I guess they felt I was embarrassed enough that they didn't have to mock me. I went back to the check-in line, which was a lot shorter by then, checked in my suitcase and got back in line for security. I was willing to take a certain amount of ribbing regarding my screw-up but again, everyone involved was polite and didn't point out how dumb I'd been.
I guess the moral to my story is when at BLI:
1) Stay in the longer line if you have a bag to check;
2) The line with TSA is not where you should be with anything other than carry-on (though I suppose that applies pretty much anywhere).