I once had this when a previous entry to the U.S. was not matched by an exit. It was the days when the green slip of card was manually removed from your passport by the airline you left the country with.

Luckily, I reentered before the original 90 days had expired so the error was spotted. Overstaying is really not appreciated and good luck telling CBP it's not your fault.

I was effectively reentered within the original period, so covering up either the CBP or the airlines failure. The following trip, 4 weeks later, resulted in the usual 90 day period being granted.