Got to work on a busted Centipede that booted to a white background screen that briefly showed the start of the attract mode (things drawn in green or purple) before crashing, watchdogging and doing it all over again. Test mode would do the same thing as well.
Disabling the watchdog (by grounding the WDDIS test point) did not help as the game would just essentially do the same things but just not reset after it crashed.
When the attract mode was drawing the playfield, I noticed that the mushrooms were all in a single line. I remember this being a sign of a bad POKEY chip, and removing, cleaning and reinserting it did not solve anything.
I was not sure if a bad POKEY could cause a Centipede to keep crashing, but I dug a spare one outta my parts and sure enough, the game came right up after replacing it.
So I learned something that day – a bad POKEY can cause more than sound problems and mushrooms in a single line.