Hm, sounds interesting. I think it might either be the iPhone itself, the iPhone app, the player is cheating or hacked into the website, (doubt it) you are lying, the website might be testing some new software in order to make the site better, or it is a major glitch or bug. I would suggest handing this over to the Chess.com Help and Support. Here is the link:
http://support.chess.com/Tickets/Submit
I would suggest you to put it under "Bugs" or "Glitches." Something that has those words in it. It is possible you will need a description of the bug, who you were playing against, and the link to that game. It is very important to include those things. I hope I can help!
-BlueGator456
I was playing chess on my iphone and then suddenly some piece disappeared! And i saw my bishop was moving like the horse! Was playing against Yus1962 (1338) and then i decided to log onto my pc and i saw that my bishop was in fact his horse. There must of been some kind of glitch or something. By the time i realized about this i had no chance coming back into the game. Dunno whether this problem is a problem from your side or maybe it has to do with the iphone itself.
(Couldnt find anywhere that said submit ticket or something like that)