This one ties into the Sierra thread. I was playing Phantasmagoria late at night. It came on 7 CDs. Completed disc 6, got a prompt to enter disc 7. Swapped discs, went on with the game. A few minutes later, it crashed. Had to reload from my last save, which meant going back to disc 6. Played on from there, right through where I should have had to switch the disc....but I didn't get the prompt! Game kept right on going without the last CD. It's a really scary game, and I freaked out!
I literally shut down the computer and wouldn't turn it on again until I called tech support. The tech was as surprised as I was, but his best guess was that because I had started chapter 7 before the crash, there were likely bits of data from that playthrough on my computer, even though I had to reload.
So I went back to the game. Eventually it asked me for disc 7, but way later than I would have expected. So weird!