I made a bit more elaborate answer so I thought I'd post it. =)
I would confirm with Naberrie that this is correct, put the counter on, and remind both players to play more carefully.
You could argue that according to policy, Anakin should receive a Game Play Error: Game Rules Violation and Naberrie should receive a Game Play Error: Failure to Maintain Game State warning, then a full backup should be performed to the moment Arcbound Ravager entered the battlefield or if not possible, Arcbound Ravager should be put into its owner's graveyard the next time state based actions are checked. However, firstly, that would be more disruptive and quite silly. Secondly, I would argue that the counter was there and both players knew it, they just didn't put it there.
So basically, my thought process is that no infraction or backup for the same reason no infraction would be given if somehow a bunch of +1/+1 counters spilled onto Anakin's Arcbound Ravager and neither player noticed right away. Both players know the board state. They just aren't physically representing it properly.
Edit: Also IMO Anakin would never have <spoiler>turned to the dark side</spoiler> if Padme hadn't insisted on playing her Stasis deck all the time.
Edited Alexander Traplin (Jan. 9, 2015 01:37:37 PM)