Blank gem positions during game

http://imgur.com/a/9emDM

Although my screen shots don't show it, the "hole" would fall like it was a regular gem. I could move the hole around like it was a gem and it would swivel back to position without matching. I wasn't able to test if it could match with many other gems because the game ended too soon.

Bug? Feature? This happened twice in the same match and I haven't seen it before or after. It wasn't horribly disruptive, but I thought it was weird enough to share.