well, if 1 bit doesn't check out, it'll generate a rom error. the corrupted data in question is probably something you won't ever see or notice, but it's still not a 100% working rom because of this.
I'm sure there could be other factors though, like voltage, which wouldn't make sense if other roms are fine I would assume. or commonly corrosion on the chip legs. you might want to pull the rom and inspect it to see if it's dirty. if there's corrosion, which will stick out very easily cause it will be black in appearance or perhaps green oxidation, some fine sandpaper will take it off.
otherwise, you're probably going to have to get a new rom, or reprogram it maybe if you have the means (and it's reprogrammable)
EDIT: this is what one of my Defender chips looked like
I'm not certain what is contained in ROM 5, but since your game is working it's obviously either data (in which case you might have sprites or backgrounds corrupted and not be aware of it) or code that isn't executed on a normal game (if it did execute this code, it would likely trigger a reset).
If the problem is just with the ROM 5 contents, you would want to get the ROM erased and reprogrammed (or just order a replacement that's already programmed). If possible, could you save a copy of the ROM contents before wiping it out? I would be interested to see what exactly got corrupted.
What kind of EPROM is it? 2532? 2732? I can burn one for you. But if you can, take a picture of your board for me so I can see what EPROM it is and hopefully what version it is. (Joust had 2 or 3 different ROM versions and color-coded them.)
What kind of EPROM is it? 2532? 2732? I can burn one for you. But if you can, take a picture of your board for me so I can see what EPROM it is and hopefully what version it is. (Joust had 2 or 3 different ROM versions and color-coded them.)