Guillaume,
I can understand some of your frustration. Based on the information you've given us, I would consider the following: If cards were properly tabled and the dealer kills the winning hand, the error should be corrected. The problem that could arise is the failure to notice the mistake by other players, or failure to bring it to the attention of the dealer. Example: At showdown, Adam has a straight...Billy tables a flush but does not realise he has the winner. The dealer also fails to recognize the mistake. The dealer proceeds to kill Billy's flush and awards the pot to Adam. The dealer quickly gathers the cards and begins to riffle the cards for the next deal. At this point, the next hand has officially begun.
This is where the debating begins. A good example for no further review would be if Billy failed to properly table his hand and then realised that he had a flush after the first riffle for the next deal...it's too late. REASON: the next hand has begun.
In the case of the dealer overlooking the winner, and other players recognise the mistake (even after the first riffle for the next hand), I would award the pot to Billy. I really believe that there are numerous rules that could be enforced on many similar situations. This is where the floor must make a decision in the best interest of the game...the integrity of the game!
As a tournament director, or floorperson, or gamekeeper, or dealer, or player...the common goal should be to assure that the best hand is awarded the pot.
When is it too late to correct a mistake? In my opinion, once action begins on the next hand. This is not a written rule, it is what I would base my decision on in the best interest of the game. This is the ruling I would make for numerous reasons.
#1 Action has begun on the next hand. #2 Proper cards have already been dealt. #3 The outcome of the previous had might effect the table stakes of remaining players. An example here could be taking chips away from Adam, giving them to Billy and possibly eliminating him from contention.
Think of the "breakdown" of players and the dealer, for such a mistake to occur. The dealer fails to properly read the winning hand, Billy fails to notice he had a winner, and the other players also failed to draw attention to the mistake!
This should be a very rare occurrence.
The rules are clear for many situations that you face every day. However, your knowledge and skill, combined with an accurate account of exactly what transpired at the table will allow you to make the best decision in the most difficult situation. Good luck and I hope this helps. I know you are determined to make every call the correct one. This speaks volumes for your integrity. You must come to the realization that there is not a rule for every situation you will experience.