villagers randomly sink into cobblestone (and possibly other) blocks, and have to be pushed out again.
tested in custom superflat (preset 1;95 and 2;95) in both creative and survival. does not occur in default type world made in 1.3.2.
edit: may be similar to https://mojang.atlassian.net/browse/MC-130
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Duplicate/Related To: MC-119