Beeping computers

No computer beeps or flashes lights unless there's a Good Reason for it to, which usually is limited to operational errors (though many do such during boot-up sequence as part of their self-tests). Generally found in fiction prior to home computers appearing (and thus in the time when ignorance of such was great) or fiction trying to mimic such (as it has its own appeal).

Especially egregious in video games as all of them have been made after the fact people learned The Truth. Oddly enough, many video games still have them.

Historically beeping and flashing lights were used in extremely early computers (such as punch card computers) where their operators could actually follow them to monitor how the computer worked, but this was quickly phased out as computers grew too fast to monitor in such manner. In modern times these are only used for simple diagnostic things or network devices that are usually not very visible (not enough for this tag certainly).