Bugs. Grotty little cretins that lodge in your windpipe, frighten girls and ruin family picnics. Eat your greens like your mother told you to and bugs are kept at bay - the medically related ones, at least - but even a prize cauliflower would wither in the face of a digital creepy-crawly, which can cause games to crash and characters to morph into lamp posts.
It's not only end users who gnash their teeth in frustration; developers also scream "D'oh!" when quality assurance (QA) goes pear-shaped.