In some cases, on SMP when inside a protected area (an area in which the destruction and placement of blocks is prohibited) half-slabs (stone half-slabs tested) could have another slab placed atop the existing one to create a 'whole slab'. This was not reverted by the server and remained there. However, other blocks could not be modified in seemingly random areas under the same conditions.

This server was most probably using a plugin to protect this area (WorldGuard) and was a Bukkit server. This server was a creative server and half-slabs were used to border plots.