Cars
I used to work in a car parts shop, and regularly encountered customers who should not have been allowed to control a car on a public road.
Hardly a day would go by without encountering someone who doesn't know what make and model of car they own, and some people didn't even know what colour of car they owned!
Common sense was often a rarity, but one chap stood out in particular. It turned out he was a secondary school physics teacher - a man charged with educating children. He came in and told me a red warning light was lit on his cars dash and could I have a look. I went out to the carpark to see a crusty old Nissan. He started it and the oil pressure warning light remained lit. I asked when he had last checked the oil.
"...oil?"
I explained that cars needed oil and his was likely to be very low on oil. I suggested he check it and fill it as needed, which would likely put the light out. I explained that he may have done long-term damage but on an old car it would probably soldier on for a while yet. He seemed happy and off I went.
He came back in ten minutes later and told me the light was now out, but there was a very large amount of smoke from the exhaust and could I come and have another look?
Out I went and spotted the three empty 5-litre oil containers.
"did you put all of that in the engine?"
"Well, most of it. Its full up to the top now"
"oh dear".
I explained the concept of a dipstick and how to check the level and told him he should let us give it an oil service to prevent more idiocy. He claimed not to have time for that and asked what he should do in the meantime so I cut the sides out of the oil containers to use as drain pans and loaned him a spanner for the sump plug, explained how to drain the oil off and told him to come and find me when it was all out. He came back into the shop ten minutes later filthy dirty, but smiling.
"its all out now"
I went outside to find maybe 6 litres spread between the 3 cans and the rest oozing across the carpark.
"sorry, I spilled a bit"
"...so I see" I filled it up for him and off I walked.
He came back 2 minutes later.
"I have another problem, Im am locked out of the car"
*sigh*
All the time we had been back and forth, he had been almost OCD about locking and unlocking the door every time it was opened or closed, and now he had managed to lock it and somehow leave the keys lying on the passenger seat. After a few minutes fiddling, I had opened the car for him (we had to do this quite often for similarly dippy customers so knew most of the tricks) which impressed him mightily. I handed him the keys and watched as he put the key in the lock and slammed the door shut, snapping the key cleanly off in the door.
"oh, what do I do now?"
"Oh Jesus Christ." *facepalm*
"I suggest you get a taxi home and come back with a spare key"
"I dont have a spare"

The car sat in the carpark for a couple of days until it was torched by the local feral kids one night.

A physics teacher ffs?
(dave likes cheese, Sat 20 Mar 2010, 13:53, Reply)