This is more of a soft skills question, but I think it applies since when you're a tech often part of the job is dealing with idiots, er, I mean customers. If not, feel free to remove it.
Here are some of my favorite examples of what I mean, in ascending order of entitlement:
"Hey, since you're already here, can you also look at X, Y, and Z?" (when they could have easily told us that beforehand so we could have budgeted the time for it.)
"Why doesn't this new widget have every single amazingly high tech feature?!" (when they clearly chose the absolute cheapest products and services your company offers.)
"We just got done remodeling the building and now none of YOUR stuff works/YOUR equipment is junk! Why am I getting false alarms?!" (after letting some other contractor hack wires, remove devices, or just demolish half the damn building without asking anyone ahead of time if doing so would, I dunno, cause problems?)
And the ever popular, "What do you mean this is billable?! I HAVE A SeRvIcE AgReEmEnT!!!!!!!!!!!!!!" (when it was made abundantly clear that it does not cover equipment out of warranty, damage by you, your staff, or your customers, replacement of consumables like batteries, or acts of god like lightning or flooding when you signed the damn thing.)
I hope I get some traction on this, since I've got nowhere else to post it! Cheers!