.25 is a hugh differance, I know, and old power supplies designed for TTL usually had some kick. But it really only matters if power supplies are shorted together, which is unexpected. I'm sure I can open the supply and adjust it down. Might have to change a resistor.
What does irk me a bit, is that I picked the Arduino Mega2560 specifically because it could run off of an external 5v supply.
Now I have to kludge a wall-wart, and redo my shield to separate supplies? I guess I'll live with it, not the end of the world.
Bob