Placing redstone dust in the same block as a non-full block will replace that block with redstone dust.

When you try to place redstone dust onto a block that already has a non-full block (Chests, ladders, buttons, torches, etc.), the redstone dust will replace that block completely. If it replaces a block with an inventory such as chests or hoppers, it will dump that block's inventory. As far as I've tested, this bug doesn't affect anything but redstone dust.