PIC 18F2550 USB Dual Power Example
Ok, I got it figured...If there is power on the Vbus, and there is no diode, the PNP transistor will shut off, since there will be a V+ in its base terminal, and then there will be no energy in the PNP, so it will turn on again, because this time the base terminal will receive a 0V level, and this way sending a V+ to the PNP again, turning it off again, making it into a cycle, causing an erratic behaviour...the capacitor will even make things go slower.