A neighbour asked me to have a look at her laptop as it kept cutting out - the vents were clogged with cigarette residues and it worked fine again when I brushed them out.
My father-in-law asked me to look at his laptop as the keyboard was playing up - I discovered a layer of sticky, sugary grot inside it and had to replace it - he still insists he hnever spilt anything on it!
I got an emergency callout to the data centre of the large government department I worked for, saying that the on-screen menus on an old, but vital system had disappeared. Yes - the brightness had been switched down.