Y E S !!!!!
I had the same problem with my MEGA Board.
avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51
Think it was caused by a faulty sketch upload. After that the board did blink the LED13 like crazy. and the errro uccured when i try to upload any sketch.
HERE IS MY SOLUTION:
make a big sketch (so that you have more time for the following actions).
- unplug the board.
- Hold down/short circuit the reset button/pin while the board has no current (is off).
- click on UPLOAD in arduino ide.
- while the sketch compiles power up the board with still pressed the reset button.
- -->RIGHT in the moment where Arduino IDE says "sketch size xxxx..." release the reset button so the board boots up....
If you are lucky and hit the right time the RX/TX leds begin to flicker and the sketch is uploaded. 
I can not say how exactly the time of releasing the reset button must be hit. For me it worked on the first try! 
I also tried the describes solutions with reinstalling Driver, differnet PC and so on. but nothing worked.
- NOW my board is like it was before. uploading a new sketch woks like desingned. no tricks or workarounds are needed.
Just for info: I'm using Win7 Ultimate 64 Bit.
Just wantet to tell you that I soled this problem. Maybee anybody finds this post useful!
Regards Bernd