I joined a world (in the latest snapshot) that was last loaded in 16w41a. I had an Etho Hopper clock (where the items go back and forth in 2 hoppers facing each other). After going to the Nether and back, sometimes the items would stop moving or move very strangely (it would send 1 item into the the next hopper and send it back without sending the whole stack of items over to then be transferred back).