Just that really. I was cutting up mushrooms in the kitchen of my & DH's first house (modern, we'd had it from new) & one rolled off the board & onto the floor.
I saw it roll & heard it hit the floor. Immediately I went to retrieve it but it wasn't there.
The kitchen was small & fully-fitted & there was literally nowhere it could have gone without being noticed. The door to the hall & the outside door were shut.
I checked inside all the cupboards, even though they too had been closed, in the sink & the bin in case it had bounced...
I've always assumed it somehow slipped through a gap in the space/time continuum 