From the other side of the counter: Having to deal with customers who were utterly convinced that the bike or product they've dragged through the door is covered under warranty when it wasn't.
Like the guy who buckled his back wheel (dirt jumping a XC hardtail.. :/ ) so decided to unhook his V brakes to allow tha wheel to turn. It promptly swung underneath and into the wheel removing half of the spokes and ripping off the flange and ripping off the brake boss which ripped through the seat stay writing off the frame. £900 bike reduced to a pile of parts and scrap in a moment. I felt for him, I really did but I wasn't about to pay for his screw up.
Best bit was trying to contain giggles when folk came in having done something really dumb - like (genuinely) greasing rims to stop brakes squealing and then complaining that they didn't work!
Taking a bike apart and reassembling it wrong - front mech on the down tube. Chain run through it and everything.
"You've put it back together wrong".
"No I haven't".
"It's supposed to go on the seat tube".
" No it isn't".
And repeat.
We had one the other day.
"My back brake is jammed on!"
"Ah, that's because you've missed a spacer out when you replaced the wheel and you've squeezed the frame in"
"I haven't touched it and the bike has been perfect for months!"
"All we need to do is replace the spacer and all will be well. Do your gears work properly?"
"No, they've stopped working, too"
"Well, if we put a spacer on the rear axle everything should be back to normal"
"But it doesn't need a spacer!"
"OK, then there's nothing we can do"
"Right, I'm taking it to a proper bike shop!"
Where he did it all again.