ZX Spectrum hardware faults
I worked briefly as a "Service Engineer" for the company that repaired ZX Spectrums that had gone wrong while still under guarantee. Most often the fault was with the power supply - it used to overheat very easily. If not, then most commonly a memory fault in one of the memory chips or a ULA issue. We hated replacing the ULA because of the number of pins that had to be desoldered and then resoldered by hand. The manufacturer didn't put them in sockets in order to save money on the build cost.
Home computers were a very new thing for most people, and the non-working Spectrums were often accompanied by letters that said "I broke it when I typed 'L' instead of 'P' by mistake". One reason why people were so scared of the new technology was just that - they were afraid something bad would happen if they pressed the "wrong" button. Only typists were used to a keyboard - everyone else was just used to hard-wired buttons that had a single function.