I have a MacBook Pro (Retina, 15-inch, Late 2013) and the 'E' key has stopped functioning altogether.
I've been quoted between 15-250 to replace the keyboard. I haven't been able to leave the notebook at a shop yet but my workaround is to use it stationed connected to a Bluetooth keyboard.
I wonder if this issue is common with my model as well (not the butterfly mechanics i beleive)