1) A person wants to buy a desktop/laptop/other stuff. They ask for professional advice, while they've already made a decision. It all comes to long and boring arguing until I either give up and agree (even though I disagree), or the person gets mad and buys whatever he/she decided regardless of my advice. This also relates to Intel/AMD/NVidia fanboys.
2) A person wants to fix something very new or really expensive, and after hearing the approximate repair price gets mad. Attempts to repair it at home, and ends up bringing it back to me to pay even more in order to fix the original problem, and the ones he created afterwards.
3) A client get's mad when I don't want to give any advice or fix anything that is older than my neighbors kid. Gets even madder when I reveal the best case scenario: "Throw this away and buy a new computer".
4) Know-it-alls , who dare to tell me how to do my job.
So, basically just like here on TPU, but sometimes there is some cash involved and you are almost always within a reach to slap a person (if required)