I found this on accident. Tried to reproduce it, but I'm not sure what the triggers are exactly.

1. First, I made my character into a heavy class to complete the tower defense challenge. I then completed the challenge.

2. Next, I sold my ability points to take points from engineer class.

3. I then changed my build to Light and entered a match.

After doing these, I was a light operative carrying around a minigun and doing fast and high parkour. I then did a little research on the subject, and it looks like the exploit is gaining ground. Playing later in the day, I encountered a light operative with a grenade launcher. Get busy fixing it up!