4 Slot M1-ROM Error

I've had the same four carts in my 4 slot for a while now. I recently took them out to clean a couple of them that were giving me random garbled music/graphics. I kept them in same order I had them in before:

1. Andro Dunos
2. King of Fighters 2000
3. Over Top
4. Samurai Shodown 3

After reinsertion, all games demos would work with sound for one cycle through, then the games would still play but the sound wasn't there. When switching from Andro Dunos to KOF2k I got the following screen:



So I switched the order of the carts around a few times and got a working combination:

1. Andor Dunos
2. Over Top
3. Samurai Shodown 3
4. King of Fighters 2000

So what the heck? It plays fine with the original arrangement of carts I used for over a year and then one day just decides it doesn't want to anymore? I'm calling shenanigans.

Anyway, with the new order I flipped through the lineup a few times and the sound didn't go out nor did the M1-ROM Error show up.

I thought my mother board might be flaking out, now I think it's alright. Can anyone make anything of this or is it just one of life's mysteries? Oh Neo Geo