Looks fine to me. The King was not in check. The move does not place the King in check. Also, the move to f3 is within the proper way the Rook moves.
I am no expert though.
Why Is This An Illegal Move?
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It looks perfectly legal, you should try reporting this to the chess.com staff using a ticket, or by email.
Thanks everyone! Will do
Apr 12, 2014
0
#6
At the top, I can see in the notations 13. Rxf3 is already registered. I guess, he has already played that move and then clicked on the notation 12...exf3, thus unable to make any further moves.
The move appears legal to me, but which piece style and board is that?
look at the notation on top, he already played Rxf3, he is just messing with you
We just all got trolled 
I'm not sure if trolling, but if you look at the game, the notation on imgur is not what happens in the game itself...
imgur game: 11.Qc3 Nf3+ 12.Qxf3 exf3 13.Rxf3
actual game: 11.Qc3 Nf3+ 12.Rxf3 exf3 13.Bxf3
http://www.chess.com/echess/game?id=87895068
(this is a game in progress btw)
So the computer showed the wrong image
The computer seems to have shown the wrong image, yeah. Never once was there a pawn on F3 and a Rook on G3 *at the same time* like his pasted image shows (and he's not a photoshopper or anything). I'm actually really confused myself and so is he. Something went wrong bigtime and this is not a troll, obviously.
Yeah once this happened to me, in live chess. I played Ne4 but it showed up as Nd5. I tried to move my knight on d5, but it wouldn't work since my knight actually was on e4. After the game I went back into the archives, and found the game. It said that I played Ne4, but in the game, it showed up as Nd5. This sounds very similar to what your friend is experiencing, and I asked the chess.com staff, it was because of lag.
Is this an engine bug or something? My friend sent me this picture of a game he is playing and said that he couldn't move his Rook to F3 because the game said was an illegal move and simply wouldn't let him do it.
He's 100% sure that the move to F3 was the illegal one, I asked him to make sure before I posted this. Is there something weird I'm just not seeing or is the game bugged?
http://i.imgur.com/5E8Vyny.jpg