I discovered this bug while constructing sZpeddy's villager trading hall. Put simply, it occurs when new villager enter the system by falling down a 3 block gap in a minecraft through one open fence gate onto a detector rail. The villagers vibrate in the minecarts. The adjacent detector rails become confused and think that the villager in the minecart is above it.

This bug does not occur with any other mob or player. If you want to see the trading hall in action go to sZpeddy's video on YouTube. This bug and repair my villager trading hall! I am playing on a Samsung Galaxy J2.