Silly things:
Many many years ago on a now defunct forum, one of the members fitted a new sound card to his PC. When he fired it up his external speakers didn't work.
Being a PC savvy kind of chap, he spent hours re-seating the sound card, configuring drivers, checking for conflicts etc.
He eventually reported success . . . when he remembered that he had forgotten to push the 'on' switch to power the external speakers.
Took him a while to live that down