windows 7 (64 bit), Java 1.8.0_31
If you place 64 or more tile entities (such like signs, furnaces, chests,...) in the same chunk by using the /fill command wherever it replaces another tile entity, it still displays the former tile entity. Sometimes it displays the tile entity you have placed and the former tile entity in it.
This is just a render bug. You can use every single block you've placed with /fill still as the block you've placed and not the one that you see. If you reload the map, everything is fine again.
This is probably related to
MC-82703 Block Entities are not properly removed when 64 or more blocks are changed at the same time in a single chunk