I didn't know what you meant there, but I checked the episode list on Memory Alpha, and indeed, the production numbers jump from 22011 to 22013. I suppose they had an episode #22012 in development but it was abandoned for some reason. Sometimes that happens -- a script in development just can't be made to work.
Gasp! What if the episode was produced, but never aired due to suspicious reasons and lost forever? What was in that episode? What are they trying to hide from us?! False flag, JFK, Masons, et cetera!