Back when I worked at a parts store I got a few "stupid customer" stories too.

1.) guy buys the smallest, crappiest battery we sold, then gives me a $10 tip for installing it (we weren't supposed to take tips, but hey... its free money)

2.) Guy comes in saying his truck won't start when the engine is hot. Headers wrapped right around the starter are glowing red hot.

3.) Guy brings in battery, I load-test it. Its bad. I tell him its bad. He says test it again, so I tell him how its bad to load test a battery more than once. He says he wants it tested anyway. So I test it again and it starts smoking and boils over "Ahhhh man you done ****ed up my battery"