In SMP, cobblestone stairs and oak doors placed by other players cannot be interacted with and are invisible.

Oak doors naturally spawned in villages are invisible to all players and cannot be interacted with by any player. Acacia doors do not appear to have the same problem. Have not tried it with other kinds of door.

Cobblestone stairs are likewise invisible to players who did not place them. Walking onto the block results in getting stuck until the player who placed the stair block removes it.

In single player, oak doors disappear after some time. Sometimes, only the top or bottom half of the door will vanish while the other half remains.