Whats going on here then??

Been happily printing away - Prusa P3 Steel, Mega 2560 and a RAMPS, then during a print the printer just stopped half way through with a 'finished' in the display and the X carriage over on the extreme right.. I just cussed it and chucked the print in the bin (as you do). 15 minutes later I noticed the display was flickering..ummm I thought, that looks like power supply issues, so I pulled the USB out, now I should explain that I have removed the D2 diode on the RAMPS, along with the Vin pin to the Mega so there is NO WAY the +12v from the RAMPS could be powering the Mega....BUT....with the USB cable (and hence the +5v) removed the display remained on and the mega powered up.....WTF!... what is going on there? nothing plugged into the Mega barrel socket, nothing plugged into the Mega USB and no Vin connection. The fault is definitely on the Mega as I have swapped it out and the printer is back in service. The now removed, defective Mega will accept programming over its USB and will still run 'Blink' but several of its I/O pins are fubar.

Its not a big deal but I wondered if anyone might have seen something like this before...

D2 is the wrong Diode to remove when you want to power the Mega from USB only. It's D1, sitting under the X-driver on RAMPS.

Sorry..I didn't check the nomenclature before posting. D1 was the diode I removed..not that it would have mattered as I had removed the pin as well.. anyhow, the Mega is duff so it's in the bin.