Here's an interesting situation: I have 2 Arduino Megas connected to each other over the Serial3 port and attempting to communicate data (yes, they have their individual grounds connected over the same comms cable). Mega 2 receives signals to activate relays, Mega 1 (a Sainsmart TFT Screen package programmed to read an SD file containing activation sequences) sending signals over Serial link to Mega 2. When I connect power to one of the Megas (whether through external power or the USB port), it feeds the other Mega whose power has not yet been connected. It doesn't matter which one is connected to power, the one not connected will have it's power LED glowing brightly.
In fact, when the relay-Mega is connected to power, the Sainsmart TFT-Mega's screen begins to glow (but not boot up), but only after a few seconds; as if the power filtering caps were slowly charging.
Power is leaking thru diodes that provide IO protection on the 2nd device.
The data sheet clearly says IO signals are not to exceed Vcc by more than 0.5V.
You have 5V signals with Vcc at 0 - now you see what can happen. Make sure to apply power to both devices!
Much simpler solution is to place a 4k7 resistor in line with the serial signal line at each Mega - this will protect against this
and other scenarios which could damage the Megas - it will limit bandwidth a bit but few people use serial at more than 115k baud
which should be fine.
Incidentally if you power a CMOS chip this way through one of its inputs you may damage/destroy the chip - it pays to be really
careful when designing circuitry to add protection to the signals coming in and going out to external boards...