Looks at dmesg... "Heh, how is this thing still running?"
I have no direct knowledge of this, but I'm sure it happens all the time. It's just that there's a lot of redundancy in all aspects of flight operations. For example, there are two pilots. One pushes the buttons, the other crosschecks it.
If you fly Air Canada, they'll have you believe that a single light in one of the three the restrooms is a critical safety concern, and that's the reason for your 6 hour delay without compensation.
If it's the smoke detector light, that is a critical safety concern.