Snapshot 13w03a - Generated a superflat "overworld" preset map. Some grass spawned on sand, and some of these turned into seeds immediately after world generation. Confirmed in this screenshot: also happens with flowers.
Snapshot 13w3a - Once again a superflat "overworld" preset map - see screenshot example #2.
Version 1.4.6 - Generated a superflat "overworld" preset map and found a grass plant floating - see screenshot.
You are correct Anon Ymus - in my example the minecart is still "on the track" because it keeps going around.
Version 1.4.6 - Minecart moving in clockwise direction appears off track at about the 6:00 position and returns to the track at about the 12:00 position.
Still present in 1.6.4 and 1.7.x
Will this be resolved in a future version?