I placed some barrier blocks, then went into them in spectator mode. I went to all the alternate gamemodes to test this, but every time, I crashed when changing gamemodes whilst inside the barrier block. This happens both on SMP and SSP. I found that when logging back on in SSP (but not SMP), the barriers will usually be gone, and this occurs when the head is in the barrier. It does not matter if the feet are or not, as long as the head is in, the game will crash.