How to properly monitor supply voltages
The problem is that once your supply is not stable, all bets are off. Any glitch that takes Vcc below the minimum threshold in a processor circuit or similar is really bad news. 

If I suspected that this was the case but was not able to catch the event with a scope (perhaps because it was very infrequent) I would consider designing an analogue latch circuit to indicate in some way the fact that the supply had fallen below limits. But this would still be to confirm the problem so that I could set about trying to fix it at source. I would not be trying to make a processor detect when its own supplies were flaky (although things like brown-out detection exist, but that is a bit different).