Minecraft 14w07a, Minecraft 14w08a, Minecraft 14w10b, Minecraft 1.8.1, Minecraft 1.8.2-pre4
In the latest snapshot, if you control pickblock a block it copies the NBT data included in it. If this is a command block, it copies the command in it. This is a great feature, but I have found a problematic problem that arises with it.
Once you have a command block with NBT data in it, anywhere you place it will have that command in it. The only way to get rid of it is to drop/delete the item. If you regularly pick block a command block, like we are so used to doing as they are not in the creative menu, it will select the command block with data in it. If the command block is on a clock, the command is immediately ran. This is fine if the command is a testfor command, but if your saved command is summon PrimedTNT ~ ~2 ~, problems arise.
Please make a regular pick block erase the NBT tags when it selects it, so no more tnt is spawned on my redstone causing me to restart.
Command Pick Block a command block with a command.
Place a command block with/or without data
Regular pick block that command block
place the command block selected and see that it has a command in it.