This is a trick a lot of ICs are quite good at: because of the protection diodes on the inputs and outputs of most chips, voltage on the pins will also pull the

This is a trick a lot of ICs are quite good at: because of the protection diodes on the inputs and outputs of most chips, voltage on the pins will also pull the internal power of the chip up if the power pins are not energised. A lot of chips will run quite happily like this until either all the IO pins supplying power go low or the IC draws enough current to damage the diodes (which aren't generally designed to continously conduct). I have on one occasion had a power IC (stepper motor driver) manage the same trick for a few minutes, which was very unusual and made debugging a lot harder (because of course the power pins are connected, it was running just a few minutes ago!)