When powering a Arduino board with 5V to its 5V pin, then it is possible that a current will flow into the computer via the USB cable. The Arduino Mega has a 500mA polyfuse a mosfet in the path. The polyfuse should prevent major problems.
If you have a very strong 5V power supply with a very thick wire and touch the 5V pin with it, then the voltage regulator might blow by a reverse current peak. If you first connect the 5V power supply to the 5V pin and then turn on the power, then there is no problem.
Because if this, some advise not to power an Arduino board with 5V pin, because it is 5V output. Others advise to use what is best if you know the risk. There are even Arduino users that prefer to run a Arduino board with 5V to the 5V pin.
My opinion is all of the above
@jremington , Hmm. I was not talking about the servos, I was talking about the USB power vs the power from the 5V pin. From @Koepel , reply it seems that the 5V pin is designed as an output. So I'll put 7.5 on the MEGA power input and 5V on the servos, with separate power supplies.
5V. This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
That is the official text. It is possible to damage the board, but only if you try really hard.
I have a small power supply with a turn-knob to select a few fixed voltages and 7.5V is one of them. That is what I often use. It is safe. For a final project I sometimes put a DC/DC-converter of 5V on a prototype shield and power the board via the 5V pin.