When connecting to a (vanilla) server using a mobile network (4G/LTE) with spotty reception, the player begins to fall. When the world around loads, the player is stuck in blocks and begins to suffocate. I've experienced this several times, and when it's happened, I've been able to verify my Y coordinates with F3. I suffocate 5 blocks lower than the ground. Each time has been in the Nether (not sure if that matters).
For contrast, in 1.3.2, when the connection was slow/unreliable, the player would begin to fall, but when the world loaded they would suddenly teleport to the proper Y coordinate.