I put in a used game disc from Blockbuster. There was a loud *crunch*, the system said it couldn't read the disc, and I ejected the disc to find it cracked beyond repair. I inserted a CD to see if it was shredding everything, but it just didn't read the CD. Then I put in another used game from Blockbuster, and it came out cracked, too.
I've noticed a red light come on a couple of times over the past few months. But the light wasn't red when the Wii broke the discs.