not bike related, but working in a hardware store for years - i have seen a bunch of stuff.
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one dude i felt bad for - his wife had convinced him to go down to the hardware store to install a new ceiling fan. honestly, it's not that hard if you're a handy-type... but this guy had the fear in his eyes. after giving him the fan braces and explaining a little bit about electricity, and giving him a box and wires and some other crap he asks "how do i know if my electricity is off?"
:hmmm: after figuring out how old the house is, i explained flipping the breakers in his breaker box etc... when i got the "where do i find that" i knew he was in trouble. i swear he left the store thoroughly convinced he was going to die.
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another time i had a dude who took WAY too much acid come in and ask me for pesticides for his bed, as he was convinced he had bugs all over his sheets when he went to bed. that was just an awkward 5 minutes.
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my favorite was when high school kids would beeline right for the plumbing section to (1) build a pipe bomb or (2) build a bong/bowl or (3) build a funnel.
"dude, do you have like the little screen that goes in the faucet? ours got all messed up, and my parents are coming home"
when? was that at the party you just had, and your buddy needed another screen because his was all messed up and he needed to pull some tubes? righteous.
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grill covers would go out like mad in the summer... and usually we would sell twice as many as we would new grills... because after grilling, the OCD-types like to clean their grill up instantly, right after those hot juicy burgers came off the grill - good idea (makes for easy cleaning), just don't try to put that plastic cover on your 400 degree grill as soon as your done cooking - let it cool off a bit first, unless you like melting grill covers.
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and my all time favorite - (and this is the truth). we got a call about a light fixture in a bathroom of a house a couple just moved into. it was a three bulb fixture over a sink. there were two light switches in the bathroom. when the first switch was put on, two of the lights in the fixtures, the lights on each end, would light up. when the second light switch was turned on, a fan went on. they wanted to know about wiring and if they could find the third light switch for the middle light, and if they needed any fancy electrical tools, or even something simple like a fish tape, to find the 'missing' switch. they had a great laugh-at-themselves reaction after i told them that instead of searching for a third light switch, they may need to just search for a new bulb, because they probably just had a burnt out light bulb versus a hidden light switch for the middle light.