You've used Serial.print() to print stuff to the Serial Monitor for debugging, right?
Processing has print() and println() functions that do the same thing, writing to the same portion of the screen where you the the mismatch error message.
I'd suggest that you add some messages to the Processing sketch to see what you are getting.
One thing you may not know is that, unlike on the Arduino, the draw() method in Processing is NOT called over and over again, as fast as possible.
It is called over and over again, but not as fast as possible. It is called at specific intervals, defined by the frame rate you are using. There is a default one (that you can change), if you have no idea what I am talking about.
Therefore, reading serial data in draw() is not a good idea. Serial data should be read in a serailEvent(Serial port)) method, instead, and the data stored in global variables that draw() can access.