So I needed a tap the other day, and upon comparing the threads to several known sizes I had on hand, I knew it would be a weird size. Around here my best chance of getting something weird is a national chain I'll not name, but we'll call it "Fasteneverything." 
So I walk in, hand them the item with threads and say "I need to know what size this is, it is probably metric." So the guy compares it to various things on the handy check-a-size, and determines it is an M10-1. "Cool, I need a tap for that, please." So the guy walks down some aisles and returns with a tap, which he hands to me. Being the untrusting type, I open the package and compare the threads, which are obviously not the same. I mention to him that I think he grabbed the wrong one, and he replies with genuine surprise:
"Oh, did you need the same size I just checked?"
True story.
So I walk in, hand them the item with threads and say "I need to know what size this is, it is probably metric." So the guy compares it to various things on the handy check-a-size, and determines it is an M10-1. "Cool, I need a tap for that, please." So the guy walks down some aisles and returns with a tap, which he hands to me. Being the untrusting type, I open the package and compare the threads, which are obviously not the same. I mention to him that I think he grabbed the wrong one, and he replies with genuine surprise:
"Oh, did you need the same size I just checked?"
True story.



Good think I didn't ask him for an alternator for a '67 T-Bird...