I got called out to a 8051 counter system that locked up frequently.It used 8 large(ish) led display to show the number , and a external watchdog.
Every time the count got large , the LED's would drop the supply from the regulator to below the watchdog setting 4.5v and reset. but because the LED's were driven by MM5450,they all were still lit up and the reset was never allowed to restart.
So always make sure you have enough current.