The one that annoys me...
Is "What PC should I buy"?
How the heck should I know - do you seriously imagine I get to play with every PC on the market, and oddly enough know exactly which software you'll run for the next decade? They always look a touch askance when I say to go to Tesco and buy the most expensive one they can afford by a brand they recognise for computers (IE Dell, HP, Lenovo etc). But for me that's good advice. Tesco can stack them high and sell them cheap, you can march back in with it if it doesn't work, and if you stick to major brands you're unlikely to hit a major stinker.
Most people are happy with this when explained to them, but some people them start getting out PC magazines and asking of the 3200mhz processor is more of a benefit than the 2800mhz processor with more RAM. Or if the Nvidia GFX card is better than the ATI one. Well, I neither know nor care so I usually say "I really couldn't say without testing them, and we're not in a position to do that". Which is the truth. Generally I'd go for the extra memory of clock cycles (and recommend an SSD if possible), but that's about as far as I'd stick my neck out as I have no way of knowing if a Laptop I've never seen is "a good one", or not.
The other one is get is people clearly trying to get me to give them (and therefore endorse) a specific answer they want. They'll say to me "Isn't the Advent laptop better value as it's got more megahertz than the Dell??". "To be honest, I've never had a good experience with Advent..." "Oh, but would it be okay?" "Dunno - I wouldn't buy one. I'd spend the extra and get the Dell". "hmmm. But the Advent is cheaper and it's a better spec..." "Up to a point - it's a lower-quality machine. And it's got a magnetic drive, not an SSD - so I'd buy the Dell". "But would the Advent be *okay*?". "Probably, but if it were my money I'd spend the extra and get the Dell...". "But we use Dells at work. One of them broke last year... So I don't think Dell's ARE good".. It's really hard at this point not to grab them by the lapels, shake then and shout "Why the F*CK did you ask my advice, if you're going to F*cking argue with every word I say and try to get me to change it to what you wanted to hear???"