Apparently Hasbro has episodes that have been filmed that were supposed to be released earlier but never got aired.

One of them being this one.

This takes place between "Return of Harmony" and "Keep Calm and Flutter On."

Discord ends up escaping his stone prison again and causes his chaos but it takes the mane 6 quite a while to turn him back into stone because they end up becoming different types of ponies.

You'll find out which become which types by reading.