minecarts with hoppers that are moving disappear randomly.  This happens in chunk builds (the minecart never leaves the chunk) as well as builds that span multiple chunks. The build can be as small as a 5x5 space or a 1x8 (neither cross into new chunks).  I can't figure out why it happens - it doesn't seem to be a chunk unloading as I will replace the hopper minecart, stay in the area, and it disappears, but a minecart in a build in the chunk right next to that one doesn't disappear (it's not consistent which disappears either - sometimes it's one, sometimes the other, sometimes both).  Minecarts that aren't moving don't seem to be affected.