"gosh, I'm getting stronger"
On more than one occasion I've seen someone lifting on a plate-loaded machine where the storage rack for un-used plates is located on the machine right near the rack of plates you are pressing.
I'll see a guy doing leg presses, or shoulder shrugs, or whatever, with one plate on each side. With each set he'll keep adding more plates to the storage rack and then do THE SAME number of reps with the new weight. He'll keep going until there are five or six plates on each side, and you wonder that he doesn't think something is up that he's doing five-hundred pounds as easily as one-hundred! Sometimes the guy will add more sound-effects with the "heavier" weight... UNNNGGGHHH!!!
Everyone, even the most clueless gym guy, notices what he's doing wrong, and you're so embarrassed for him you don't know what to say or where to look.