First off I do not nor never had to work tech support hotlines or anything of the like, but I know some fellow students who work for a similar service for our school.
One common complaint:
Some one calls in on a cell phone that their computer's internet and the land line in their room are not working. Tech guy heads over and unplugs phone line and Ethernet cable and then plugs them back into their correct slots, which from what I remember have little pictures of a phone and a computer beside the respective slots.
Some others I've heard of from different sources online:
A Customer: I can't turn the computer on, the foot pedal doesn't work.
Tech Guy: Foot pedal? Our computers don't come with foot pedals?
After a bit of questioning, turns out customer thought the mouse was a foot pedal for turning computer on and off.
Another customer: I like the computer alright but I was wondering if the cup holder was covered by warranty as it broke off the first time I used it?
Tech Guy: Wait... What cup holder? Are you talking about a third party product?
After some questioning turns out the CD drive was mistaken for a place for beverages.