Board Thread:Wiki-Related Discussion/@comment-3202633-20150315034710/@comment-25733960-20150315060557

But when they posted on twitter the title of Fall, it said is was 4x09

(I think) There are 2 explanations for this

-When they wrote Fall, they didn't know Smash The Mirror would be to be a 2 part episode (4x08 and 4x09). They fixed the numbering later. But, if that's the case, Operation Mongoose Part 1 should be 4x22, but according to what they posted on twitter it's 4x21.

-Smash The Mirror is just 1 episode (4x08), but they already said there will be 23 episodes this season. That would mean there is another episode after Operation Mongoose Parts 1 and 2.