OS X 10.11.6 and Radeon R9 M370X 2048 MB
Broken: 2.78 63b3884
Short description of error
There's an operator "Mirror" that you can access while you're in the Dopesheet. I mistook this for the same functionality as "Mirror Keys," but it's different. Using Mirror will crash Blender. Simple file
Exact steps for others to reproduce the error
Simply hover the mouse over the dope sheet, hit space, type "mirror," hit enter and watch what happens.