Poker Hall of Shame X

At the Venetian, Myrna deals me AcKc and I end up in a pot with 2 other players. The flop comes

9s 9c Tc

It is checked around and the turn is

4c

But wait a minute! The imbecile who is first to act claims that he never checked. Myrna then calls the floor and explains that she didn't know he had a hand. Floor rules that the next (river) card will be the turn card and that the 4c will be shuffled back in the deck and a new river card will be selected. What happened to "significant action" having taken place?

Also note that the imbecile is in Seat 10, I am in Seat 1 and the last to act (LTA) is in Seat 2. So it's not as if I acted out of turn. I only checked when Myrna turned and pointed to me. Needless to say, Seat 1 rarely can see what Seat 10 is doing and relies heavily on the dealer for prompting.

New turn is 6h.

Imbecile checks, I check, LTA checks.

New river is 4d.

Check, check, check.

Both players table KTo and split the pot. Myrna sees my AcKc, sighs and apologizes to me. I say nothing.

Myrna was not paying attention and cost me money. A good dealer should know that there were 3 players in the hand. Had this occurred in Bobby's Room, it could have meant instant dismissal. I am not as critical of the floor decision as I am unfamiliar with the exact criteria for applying "significant action".