I have the same problem: suddenly it became impossible to upload a new sketch to my Mega 2560:
avrdude: verification error, first mismatch at byte 0x0000
Depending on the sketch, some byte being unequal to 0x14.
I did not try the loopback test, but the sketch that is in the Mega still gives serial output from a previous version, so I do not think that is the problem.
Re-burning the bootloader seems complicated and does not always work, judging from threads that I looked at. So I think I will waste some money instead of time and continue with another Mega.