Here we go again with another debatable rule. What happened to the good old days when a player made a bet, and the next player asked the dealer how much the bet was.....This game is getting too technical IMHO. What happened to the general rules of etiquette : Players should be certain that their bet is clear to all. If the first player acts with an incorrect amount, it should be corrected before others have acted. I will be real surprised if that new rule #89 (WSOP) is used after this year.
Going to chime in here because I think the terms "Accepting the action" and "Gross misunderstanding" are not being utilized correctly and are getting somewhat distorted. You're the guinea pig here Nick

Almost everyone who replied to this thread is under the impression that Player A's bet is unclear because he stated an amount of 2900. His bet is not unclear. His bet is "All-in".
After you announce "All-in", the amount is a mute point.
Then Player B announces "Calls the all-in". This is where the rule #89 comes from.
Player B is accepting Player A's action of "All-in" regardless of the amount. That's why we use the term "Accepting the Action".
Now, did Player A misrepresent his chips? Maybe.... maybe he miscounted. Still his bet was "All-in" and Player B did not ask for a count and his bet was "Call".
Therefore, there's no need for the TD to ask themselves if there was a gross misunderstanding of the bet. There clearly was not. The bets were "All-in" and "Call".
Now what constitutes a "Gross misunderstanding"? This happens when a player pushes forward a bet and it is miscounted, misstated, or otherwise unclear and subsequent action behind the unclear bet is not equal to the actual amount bet. Long sentence for a simple problem. Here's a simple example that happened recently where I made this ruling.
Example 1: Player A bet 1600 (3 $500 chips and 1 $100 chip). Player B asks the dealer how much the bet is and the dealer says "$400". Player B announces call and puts 400 into the pot (4 $100 chips). Player A corrects the dealer that the bet is 1600. Player B immediately grabs his chips back and wants to fold. Dealer calls for floor. I let Player B take his chips back and fold. The bet was both unclear and a gross misunderstanding 400 vs 1600.
Example 2: Player A bets a couple of dirty stacks of $100 chips that appear to be 25 high. Player B asks how much? Player A states $5,000. Player B says call. Player A wins. The dealer breaks down the stacks to verify. The dealer finds 2 $5,000 chips mixed within the stacks of $100 chips. The bet turns out to be $14,800. That is a gross misunderstanding, 5,000 vs 14,800.
As for Rule #89 of the WSOP rule book it is certainly going to be used for a very long time. It's been in use ever since I've been working for the WSOP and that's 6 yrs now. The only difference is the WSOP began publishing it's rulebook online for all to view and mostly became necessary as a back-up for major decisions that have happened in recent years. Most notably the cell phone ruling involving David Singer. This incident changed a lot things rules wise and was one that prompted them post the rules online as well as the Brandon Cantu incident.
That's my 2 cents on this one.