You can drop through a 1 block high floor by pushing a minecart off a track then jumping in, using a bow to break the minecart after which you fall through the floor. Both in survival and creative. Only tested in 1.6.2. Would be bad on PVP servers. Video below.
1. Place track on the floor that has 2 spaces empty below it.
2. Push cart of the track
3. Destroy the track.
4. Get in minecart
5. Destroy minecart with bow and arrow.