On a multiplayer survival server we've been playing on we've been having problems with certain air blocks being unbreakable.

we are unable to place blocks in this spot. pushing it with pistons doesn't  work. mobs and players can walk through it, so it's not a ghostblock. but when a fallingblock enters it it gets stuck in "entity mode" and it becomes unbreakable until we remove the block under it so that it can continue falling. palcing 2high blocks under the air block also doesn't work. also water flows arround it, as if it is a barrier block.

the only way we've found to fix it is with a /setblock command, even in creative we can't place a block.