Probablythis is the Faucet.
It could also be the pool, which sends many little transactions that are dependent on each other. Ideally it would send them all in one transaction.
It goes without saying that a tx can't go in a block if it's predecessors are not in blocks.
The protocol allows for a transaction and its predecessor(s) to be in the same block. The priority mechanism just prevents it from ever happening (for miners running mainline Bitcoin).