Had player A said "Call" before throwing out the 500 chip, I would have let it go, ruling it an assumed check and call.
For the record, in this case, player A did confirm that he did NOT check, but said he didn't realize that the action was out of turn and really thought he was calling.
FYI: I was the dealer. This was just 3 nights ago in my pub poker league, where I am known as the rules guru (or sometimes the rules Nazi). I ruled it an opening bet of 500. When player A complained, I asked the lead dealer for a confirmation ruling, kinda like calling the floor. She agreed with me as I knew she would since she's pretty good with the rules too. (FYI: Player B called the 500, it was checked down, and player B won.)
Player A was playing again last night and again complained about the way it was handled. I'm sure if he had won that hand, we never would have heard another thing about it.
Anyway, this type of thing happens often, but usually the player who got skipped doesn't act immediately, almost like he's wondering what happened. I keep it simple by asking, "Did you check?" When the response is no, I ask, "Do you want to?"
In the 8 years I have been dealing, I can't remember any case where the player needed additional information, or an additional ruling before the hand was completed, until Tuesday.
Normally, after the hand is over, I'll explain how if the skipped player did anything other than the check, the OOT player would have all options opened to him. I also explain how I can't volunteer that info during the hand since it may influence the action. I'm certain that player A has heard me explain that in the past. He's also one of the most vocal complaining about me being a rules Nazi.
So last night, when he brought it up again, I calmly explained that although we're playing 'free poker', we try to adhere to regular casino rules so that the players can learn and won't make a mistake when they're in a casino. His reply was that he never makes mistakes. Sigh.
I chose not to debate him further. I similarly chose not to tell him that he is also one of the players who often acts out of turn, and puts out multiple chips expecting change without saying call. I also chose not to follow up with the argument that knowing the rules helps you to identify and exploit other players when they make mistakes.
Anyway, thanks for confirming my ruling, and letting me rant.