Villagers seem to get stuck whenever they enter a block with a head or a pot placed on it. This is even happening on blocks with more than one block of air above them. Once a villager is on such a block he just rotates, jumps or simply looks into different directions. After some minutes he possibly manages to get back down. The player has no problems with moving freely on such blocks. Quite often, there are even more than one villager stuck on the same block.