Maybe get an exchange at the store you bought it from? It hasn't been 30 days since SF4 came out already, has it?
I had a similar, yet really stupid scare myself one day. I forgot I put Gears of War disc below the CoD4 inside the CoD4 case. I didn't notice that I grabbed 2 discs, and to my surprise both of them fit inside the tray with the 360 standing vertical. I still don't know how I didn't notice that I somehow placed 2 discs in the tray. It made the most disturbing noise, but I'm lucky none of them became scratched and still work.