There's another applicable rule, 53-B:
53: Action Out of Turn (OOT)
A: Any action out of turn (check, call, or raise) will be backed up to the correct player in order. The OOT action is subject to penalty and is binding if action to the OOT player does not change. A check, call or fold by the correct player does not change action. If action changes, the OOT action is not binding; any bet or raise is returned to the OOT player who has all options: call, raise, or fold. An OOT fold is binding. See Illustration Addendum.
B: Players skipped by OOT action must defend their right to act. If a skipped player had reasonable time and does not speak up before substantial action (Rule 36) OOT occurs after the player, the OOT action is binding. Action backs up and the floor will rule on how to treat the skipped hand given the circumstances, including ruling the hand dead or limiting the player to non-aggressive action. See Addendum.
NOW, strictly-speaking this rule isn't quite met because there's been only one betting action OOT (the opening bet on the river). So ultimately this case must be decided by Rule 1. As background, when this rule addition was adopted in 2013 at the Venetian Summit the attendees debated whether to consider the intervening deal as a separate action towards substantial action. If that were adopted in this case, the deal of the river would be one action and the opening river bet would be a second action with chips. The concept of the deal as an action was not formally adopted but nothing prohibits a TD of making that interpretation under Rule 1 on case-by-case basis.
SO, Case 1, the dealer overlooks Player "Last" on the turn, rapidly burns and deals the river, then "Last" immediately speaks up just as Player "A" is making the first river bet. In that case I would probably consider the river card prematurely dealt.
Case 2, Player "Last" has reasonable opportunity to act on the turn, he's contemplating (he assumes the dealer realizes he's in) when the dealer inexplicably burns and deals the river, Player A snap bets and Last immediately speaks up. In this case I'm probably favoring holding Last to non-aggressive action on the river though some might kill his hand.
Case 3, same as 2 except Last has reasonable opportunity to speak up on the turn and to speak up and stop the bet on the river, meanwhile keeping his cards covered. In this case I would likely consider Last's cards dead. The key factor for me in this case is he had reasonable time to stop the river bet and didn't.
Thoughts on these cases, what do you base your Rule 1 decision on here?
Thanks for the interesting case! BTW, I'm changing title of thread from "significant" to "substantial"
Case 2, Player "Last" has