It depends, for me actions and such are all fair game to say "sorry, but the opportunity has passed." since the actions, while important don't have a big wide impact upon the game as a whole. Things like Rebel Captive and the likes MUST occur, they are not a case of "Sorry, missed the timing!" because the wording is not optional...it happens, like it or not.
What I do think is sometimes acceptable is to give the player a break when it comes to massive game changing things, usually this is in relation to flying someone off the board. I once saw a game at a tourney where one person set his dial wrong and went flying off the board by his own hand. He looked at his opponent and his opponent just shook his head and said "x points to me." Frankly this disgusted me as a person, it was the opening round and yes while the rules do favor the opponent, I did not feel it was in the spirit of the game to get a one up on your opponent due to this cataclysmic mistake.
I get that people says "Then he will LEARN!" and yes he will learn....but not if he quits the game because of the over competitiveness of a community.
You can't take back every little mistake, and if it were 4 rounds in and he thought he might be able to skim the edge but he visualized it wrong and ended up just off the board, that is fair game because the intention was that he was going towards that board edge and ran that risk. However in that instance it was a clear mistake of direction that no sane mind would have made.
This also applies to red maneuvers when stressed. I've never been in that unfortunate situation, however a friend of mine has (which I admit may introduce bias into my account). This was in the days where double falcon was utterly dominating events. The opponent of my friend was indeed running a double falcon and, naturally it was the favorite to win, and it was beating my friend in this game. At one point the player missed an opportunity for an evade action and my friend let him take it as he said that he felt it was the sporting thing to do. So he gave his opponent the action, didn't really effect much, one single shot missed on the falcon is pretty common. Anyway, later into the game, my friend was already down a ship or two, and then he performed a K turn with his Y-Wing while it was stressed.
Rules are clear on this, but he discussed it with his opponent...who did not let him alter the move (as per rules) and took the dial...his opponent then obviously thought that since he was CLEARLY at a disadvantage with double falcon...that he would attempt to fly that Y-Wing...which was pretty lackluster in itself, off the board. I feel vomit coming up as I say that. While it does follow the rules, it breaks the spirit if competition for me, not only did he not return the sportsmanship, he also attempted to instead of putting the Y-Wing in a bad situation where it would not have the falcon in arc and the falcon could get a range 1 bonus on it, he instead decided it was more sportsmanship to simply eliminate the Y-Wing from the game with minimal effort on his account. THAT I find disgraceful. It is not surprising that this player (who will not be named) went on to win said event with dual falcons and while my friend doesn't seem to hold a grudge against him. I do, and it is exceptionally hard as a commentator for the X-Wing events at our local to remain impartial when commentating his matches because I just feel he's a bad player. I know that's me saying I don't like the way he plays because its different from me.
Just to be clear, i have no qualms about choosing your opponents maneuver for them, my problem is when instead of simply putting them in a bad position where they have a small chance of coming back into it, you take the quick and easy path and just send them off the board, which I feel is lazy, and doesn't show skill in your ability to be a player at all. Everyone says "Oh, it's a game about positioning, getting into your opponents head, and knowing the best combination to get the maximum efficiency out of your list." they don't say "It's a game where you wait for your opponent to screw up so you can save your dice rolls and just send them off the board."
That being said, I am not against sending someone off the board if it requires effort. One match i had with my A-wings vs a Decimator, Firespray and Shuttle list, I maneuvered my A-Wings in such a way that the firespray was forced off the board, however he could have picked a higher move and bypassed me. I still relied on my skill as a player to predict where he would go, and use that to position myself so it would make an area of his base fly off the board...but it could have just have easily failed.