The OAM data shows up on the data bus, so a cart can interact with it. I think that would be a neat way to copy data to the mapper, but that's for another topic.

In the past, I had OAM corruption when I used an EPROM emulator on my NES, I never was certain but I was thinking it was because it was reading it through an 74HC buffer, that would be outputting data much faster than a ROM or RAM would (like 15ns or something). And yeah in that case, resistors in series with the data bus fixes that. The glitch on that was much more erratic, it would only affect one sprite but it seemed to show up in almost every game, though occasionally not, on the same games. On the GTROM board there is a 70ns flashROM, it only outputs when the PRG/CE line is low, so I can only imagine there is something odd with that signal. I know from talking to kevtris that the NES pretty much changes into a different kind of timing pattern when it's doing the sprite DMA, so maybe it's related to that?
I haven't tried adding series resistors to the GTROM board, though I'm pretty sure it would fix it. It would prevent the cart's ROM from winning a bus conflict.
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As an aside, if you're willing to answer, how much is proper gold plating on the card edge adding to the cost?
In the beginning of this year, laws in China changed, restricting use of arsenic for gold plating. But AFAIK, that's the only way hard gold plating is done. So it cost quite a bit more than I was expecting, I'd estimate it added about $2.00 each compared to what it would have cost a couple years ago.. it's something like a 3x~4x increase in the cost of the board. And that was with the manufacturer subcontracting that part out, the quote for them doing it was something like 8x the cost. I didn't get a lot of quotes for various options, but another manufacturer that offered to do it with 5 microinches of gold (vs 30) was maybe 40~50 cents cheaper. Immersion gold is much cheaper, but I believe it would wear off too easily. The IPC specs say it's good for 5 insertion cycles. 30 microinches is supposedly good for 1000 cycles.