IDIOT SIGHTING: I worked in a DIY store. I guy asked a coworker "which is whiter, masonry paint with a 10 year guarantee or masonry paint with a 15 year guarantee?
IDIOT SIGHTING: Same store. A guy handed me and old spray paint can and asked if we had any in stock. I took him to the shelf and saw that the label design had changed so I had to examine each of the cans that we had. I read the bar code number off the can and compared it to the labels on the shelf edge. "There you go" I said, handing him the new can. "How do you know its the right one he said". I said "I read the barcode". He said "WOW how did you learn to do that?!"
IDIOT SIGHTING: I once spent half an hour explaining to a rather confused woman how and why fuses work. She needed a fuse for her bedside lamp as the one in her plug had blown. Instead of taking a replacement 3 amp plug she opted instead for a 13 amp so that it wouldn't blow next time