Several times in the new snapshot I have fallen into the solid block below me. Unfortunately it seems to be pretty random so I wasn't able to recreate it yet, but it's fairly common. I have had it three or four times so far since I started playing the new snapshot yesterday. It has happened on both grass blocks and stone blocks, so it isn't something to do with the block type. Otherwise I have no idea why it is happening. Also sorry if this has already been reported, I'm sure it probably has but I tried searching for it and nothing came up.