Bad chips even if verified?

Got a weird Q. So im using a Clemens board to make a TLROM game (specifically the mmc3 51555-2 variant). Never had a problem with it or even making the same game with it.

The problem im having is when I booted the game up there were obvious graphic problems. But it came and went. On some parts of the screen it looked fine and others, messed up. Seemed like a trace was bad on the CHR. Check it, nothing. Cleaned the pins, used my multimeter. Nothing. Same problem. So I lifted the chip out and grabbed another 51555-2 board. Same EXACT problem! So it cant be the board, gotta be the chip. Took both out and BOTH the PRG and CHR verified. What the heck is going on? The traces on the other one are perfect and id have to mess up the same address or data line on both to achieve the same exact result.

So can a chip verify and STILL be bad?

Could the chips you have be a little too slow for the NES?

I doubt it cuz i bought em with the same batch as others and they were all fine.

The only thing strange about this one is that I think there was a scratch on the top of the glass cuz it took a LOOOOONG LOOONG time to erase fully. After I fully erased it I burned it and got this problem. Specifically the CHR chip.

Yes, very common. Especially if you erase them too long, it'll cook them. Chips work like this: 100 charges or more=0, 20 charges or less=1. When you cook a chip bad, or it wears out because of age, you can program to 50 charges, lets say. It may verify right, but the result will probably not be consistent, or the programmer will be more lenient. I've had it happen with bad chips a ton, very common.

Ahhhh! Ok that makes sense, thanks a lot! This was really confusing me haha

"overcooked" EPROMS have less charge as 3gengames mentioned. A side effect he didn't mention is that since the charge level is lower the sense circuitry on the EPROM takes a longer time to detect the stored value. In simple terms a overcooked EPROM read a LOT SLOWER than it's rated speed.

Appreciate all the info. You guys were def right. As I got new 010s today and it worked fine in those boards. So it was overcooked. Thanks!!