I sure have done that - but I can top your story!
Cut out fabric for 9 big blocks. Sewed 9 big blocks. Went to put them together, got to the last block and couldn't find it! I hunted high, I hunted low. I ransacked drawer and cabinets and then did it all again.
Then I spied my little wastebasket, which I keep on my right while I'm sewing. I dump threads and scraps in it, and had about 2 weeks worth of both in it - the night before. I remembered handing the plastic bag liner to my DH the night before so he could put it out with the trash.
You guessed right. I didn't check it for 'slippage', as in 'a block slipped off my lap and into the waste basket'. As luck would have it, the trash had already been picked up.
The block is gone! I patched together another block, but it isn't exactly like the others. It's a baby quilt, so when it's finished I'm going to call it "One of these blocks is not like the others..."