Here it was all ok, Philidor position, I was going for a draw. When I refreshed my browser it showed that his king moved on the fourth rank. I moved my king too, from h1 to h2 and clicked "submit move". After the window refreshed I saw that his king didn't move and his ROOK emerged from nowhere like a F 117 stealth, taking my rook en prise.
I am pretty frustrated with that. I installed all firefox extensions for the site two days ago could that be the reason for wrong position appearing on the board.