Bukkit

When mobs heads are broken they all respawn as the skeleton head.

Paintings and item frames don't produce a placeholder or respawn.

Blocks don't keep their orientation when they respawn (chests, dispensers, ect)
*Signs do keep their orientation as it is a part of their data value I think.

Only the part of the bed that was broken respawns (which after some amount of time may automatically break
and drop a full bed item)

If the bottom block of a door is broken it will drop the door block (sometimes) when it respawns
or may put an invisible bottom part of the door in its place,
if the top block is broken it will respawn the top block of the door invisible and in place.
*The top part of the door/bed will dissapear when a block updates next to it.
*The bottom part of the door/bed will drop the item when a block updates next to it.

Wheat does not respawn but does create a placeholder.

Signs lose their text when they respawn.

Breaking the back of an extended piston will cause the extended part to dissappear and when it respawns
it will not have the extended part of the piston attached.
*unpowering / powering the piston will cause the extended part to reappear.

Breaking the extended part of a piston causes the back part to dissappear and when it respawns
it will not have the back part of the piston attached.
*unpowering / powering the piston will cause the extended part to dissappeare as well.

When redstone singal producers (torch, lit wire) respawns it does not provide power.