I don't get mad at the waiter for making a mistake, because I don't know him, and this might have really been a one in a million thing for him, in which case, it just happens. I do get mad at my girlfriend, though, for example, who always seems to bump into things, or who doesn't think very far ahead to anticipate things, or notice things, or remember things as well as I do, etc.
I guess that she can't help it any more than she can help a random occurrence, but I can't help but think that the situation would be greatly improved if only she just paid more attention to things around her.
Anyway, it irks me when people have difficulty doing things that seem trivial to me. It irks me precisely because I don't think I'm better than them, and thus am confounded as to why they seem to have more difficulty. If I thought "I'm just smarter than them", it wouldn't irk me any more because it's an innate difference that can't change, but I don't believe that...