When attempting to place doors as a non-op on in the spawn-protected area of a server, an invisible block where the top of the door would be appears. It seems like you have to attempt to place three doors before the block exists, but then all the previously placed doors will create an invisible block. It is impossible to go through the invisible block and it disappears upon relog. As a non-op, it is possible to break this invisible blocks, and they have door breaking particles. I have only tried with wooden doors.