See the story and photo from the Pokerati blog:
I have seen a fouled deck exactly one time, and it wasn't nearly that dramatic. I was playing a tournament at Caesars. They were using WSOP-backed cards. They alternated between two decks with the dealer changes. The decks' backs were identical except that one had a very thin black line around the border, the other an equally thin red line.
We had been playing for a couple of hours at the table when I noticed that my two down cards were one with a red border and one with a black border. I pointed it out to the dealer, who called the floor. It was a fouled deck. But nobody had taken any action, so it was trivially easy to fix. They took the decks away for inspection. I don't know what they found. Maybe there was an extra card. Maybe one card from each deck got into the other. But we had played with those cards for two hours or so without anybody noticing anything amiss.
Lesson: If you're going to have two decks in action at a table, they should be extremely easy to distinguish at a glance.