It's the third annual mashup episode, and while the songs may be harmonious, the relationships are not!

There's a lot of conflict in this one! New Directions vs. The Troubletones, Sue vs. Burt, Santana vs Finn, and the slap heard around the world . . . and it wasn't even in the script! Hear behind-the-scenes scoop on the scene that still makes them gasp.

Plus, "Mash Off" featured the show's 300th song and a return to the music charts with what Kevin and Jenna think is one of Glee's top three songs of all time! Will you agree? Click to find out!