Occasionally, when near an automated farm I built, the game fails to save, hanging forever during the save. When I hit Alt-F4 and then come back, the rail hopper seems to have vanished.

This is just a simple rail running under a wheat farm, with a hopper cart collecting dropped wheat and then dropping it through additional hoppers under the track to get it into a stationary chest for retrieval.

It is infrequent enough to be hard to trigger, but the same thing happened each time.