Not so much a "stupid tech question", but I'm surprised no-one has posted this already:
People who don't read message boxes or attach any importance to them.
"It then said something on the screen, I can't remember what it said..."
"Hmm, a message has popped up. I'll just click that X in the corner..."
Occasionally I can forgive them for it, for example the typical post-NT4 BSOD message with a lot of cut n' paste text with a tiny amount of useful information for troubleshooting the problem. I'm sure that it does happen that someone connects up a new piece of hardware and a BSOD results very soon after, but not anywhere as often as not.
Speaking of 800x600 - I visited a customer once, a father with an approx. 18-year-old son. 1280x1024 LCD running in 800x600. I change the setting, show the father and ask if that's ok, whether he can still read the text ok or whether he wants it a bit larger, and show him the difference when looking at web pages. He approves and very much appreciates me spotting that (the son is present and says nothing). The appointment goes on, and I recommend replacing their Internet Security product with plain free anti-virus as the former was slowing things down a huge amount. The father says ok, then his son goes off on one saying is he sure about this with a "it's your funeral" tone, and I'm thinking "you're the one who has been using a PC at such an obviously low resolution that it must have routinely slowed you down and looked crap for years, then suddenly you know more about computer security than the guy who does this for a living?".
Everyone in the computer industry must of heard the word perfect story but just in case someone hasn't
http://www.waynesthisandthat.com/response.htm
Amusing, but not a bright thing to do. The predictable result is that the customer complains that the person who answered their call was abusive and unhelpful and recounts a conveniently shortened version of the conversation.
The other semi-related customer peeve I have is that I explain why something went wrong. Yesterday it was a hard disk that's on its way out and covered by the laptop's warranty. I explain that there are various things that could cause a disk to die, personally I think that a lack of ventilation is one of them, especially if the laptop is used on a soft surface which blocks the cooling vents, but then adding that disks can start dying in many different ways and ages for different reasons, such as a sudden shock while the disk is running. I overheard the customer saying to their kid (18-25 y.o.) why it died: "It overheated". If she just wanted to give a quick answer, then "shit happens" is a heck of a lot more accurate! Although this situation might be more appropriately filed under "when parents want to blame their children for something going wrong because the child uses the item in a way that the parent doesn't approve".