I have a really low threshold for stress, and that can come out as anger and frustrated when I'm in a situation where I feel I have no control.
We have these self-check out lines at the local grocery store, and they malfunction 9 times out of 10, but I always choose that over a human cashier. By the time I've trudged through the store, which is always way too hot for me, and make it to the check out, I'm already all edgy and sweating and I'm dying to get out of there. So OF COURSE the stupid machine starts messing up and nothing will scan right.
This is when I turn into a giant bitch. Some poor clerk has to come over and override the machine, and I'm always so pissed off at this point that I can't help but give them a big attitude about it, even though it's not their fault.
I'm sweating, self-concious, stressed out, and now I'm angry about every little thing that's making it worse.
I think this is because I never learned how to cope with my anxiety levels when I was a kid, so they stay bottled up until something aggravating like this happens.