Another possible reason for this error for the Mega 2560 is if your code has three exclamation marks in a row. Perhaps in a recently added string.
3 bang marks in a row causes the Mega 2560 bootloader to go into Monitor mode from which it can not finish programming.
“!!!” <— breaks Mega 2560 bootloader.
To fix, unplug the Arduino USB to reset the COM port and then recompile with only two exclamation points or with spaces between or whatever. Then reconnect the Arduino and program as usual.
Yes, this bit me yesterday and today I tracked down the culprit. Here is a link with more information: http://forum.arduino.cc/index.php?topic=132595.0