What happens when two digital circuits are connected and one is powered but not the other?
If your logic supports the Ioff feature as implemented by TI and others, you can safely power down one side without doing any damage to the other side, provided the applied voltages are within the limits specified.
From that thread:
The Ioff feature is for isolating the device when Vcc = 0. The 10 mA is the maximum amount of current the device will input/output while Vcc is 0V. (This refers to the specific device being questioned).
The official definition:
Ioff - The maximum leakage current into an input or output terminal of the device, with the specified voltage applied to the terminal and VCC = 0 V.
The Ioff protection circuitry ensures that no excessive current is drawn from or to an input, output, or combined I/O that is biased to a specified voltage while the device is powered down.
This is an incredibly useful feature and I try and make sure I use these types of devices in power sequencers (in particular), where the power state of some circuitry may be unknown for a short amount of time.
HTH