Pinball machines have a bunch of switches in them and if a single one fails then the game can get way less fun.

Example, hit a particular ramp to get a jackpot during multiball, ramp switch breaks, now there's way less point in getting the multiball, half the game rules go out the window. Very frustrating.

Williams WPC games of the 90's had a feature where they'd keep track of whether a switch changed state over a number of games; if nobody made f'rinstance the ramp shot in several games then it'd try to alert the operator, but it'd try to do so in a subtle way that only the op would notice, rather than a big flashing "PLEASE CALL ATTENDANT AND MEANWHILE DON'T PUT ANY MONEY IN ME" sort of deal. They called this the Credit Dot, which would appear as a full stop after where the game showed the number of credits on its display, for example a properly-working game would say

CREDIT 0
INSERT COINS

but a game that had a complaint for the technician would say

CREDIT 0.
INSERT COINS

The credit dot would show up when games were set to free play too, instead of CREDIT 0. INSERT COINS it'd say FREE PLAY. PRESS START

The credit dot would also appear when a switch that was supposed to be normally open was detected as closed for a while or vice versa, or in some other situation where the machine had been treated so roughly that the computer noticed something was wrong and was asking for care.