Just tried and couldn't reproduce that. What I did was:
1) With Galileo connected to the power adapter and USB port open the IDE and serial port monitor;
2) Unplug the power adapter. Galileo happily continues to run on the USB power :-D Uplug the USB cable. Galileo switches off, Windows makes the sound notifying the COM port (USB device) is gone and it disappears from the Device manager;
3) Plug the power adapter back in, plug the USB cable back in - in a while Windows finds port again, if the number has changed - I'm able to select it in the IDE.
I've tried it several times and not always it continues to run on USB power at step 2 (confirming the documentation warning to have it powered through the adapter at all times), but that didn't change the effect on the IDE.
Care to describe it step-by-step to see if I could reproduce it? I think it would be easier to push this through as a bug if it's reproducible :-)