Left half of MacBook Pro keys periodically stop working

I use my MacBook Pro at work plugged into a Cinema Display with an external and mouse a majority of the time. Sometimes when I unplug my laptop and go to a meeting, the left half of my keyboard won't work anymore. I'll be at my screensaver login screen and typing any key on the Macbook's left side won't type anything into the prompt, but typing the right keys will.

The only thing that will get the keys working again is by force shutting down the computer and restarting it. Putting it to sleep or unplugging/re-plugging into an external display isn't enough.

I'm on a 13-inch (Retina, Late 2013) with 2.8 Ghz Intel Core i7, upgraded to latest version of Yosemite (10.10.4).