Remember that throw you turfed in that big game? That throw you "know" you have? What happened?! You warmed the throw up, you practiced the throw all year, and yet there it goes out of bounds. Maybe you got unlucky. Maybe you are 95% with that throw and you happened upon a 1/20 shot of turning it over. I call bullshit.
More likely, you're not practicing the entire situation. All kinds of stuff is different in a game. You're tired. You just caught the disc and are slowing down. The marker is striking and you have to speed up your release and pivot hard. You're throwing to a moving target.
In my opinion, you have to work hard to practice game time situations when you're throwing. Great, you can jack it 70 yards with no mark while throwing from your hip. But can you throw a 70 yard huck in a game with a marker?
These are some things that have helped me improve my throwing percentage in games:
1) Throw off of a fake. Often you'll throw a forehand off of a backhand fake (or vice versa). Do this when you're throwing. Or off a double fake. Practicing different fakes taught me that the backhand-around fake -> IO forehand is a very tough throw and this helped my mark a bit.
2) Pretend you have a mark. This ties into the first point. In particular, if you're working on your around break-mark throw, you may need to practice a different release than your typical outside in throw. Sometimes, you may be stepping slightly backwards to avoid your marker, you may throw it off of an IO fake, or be pivoting from the open side.
3) Throw off of movement. Especially when practicing hucks, I find it helpful to toss the disc in the air, take a few hard steps, catch, and throw immediately. Most of the time when I huck in a game I'm throwing very soon after catching the disc when the marker may be out of position.
4) Throw while tired. During rest time of a track workout is perfect! Get some reps in while your HR is high (like 140-180 bpm or so). This will teach you great focus for those hell points.
5) Picture the throw. Picture exactly how you want the disc to travel and not just where you want to throw it. What arc? What speed? What height? What release point? How fast do you want to get the throw off? All of these factors can be critical in a game and need to be practiced. Picture the throw before you do it, and judge yourself. Don't be satisfied if you hit your target but not your flight path.
When we do focused throwing, challenge yourself to make hard pivots/fakes on the 10th throw of a set. If you mess up, you start over. But that punishment pales in comparison to turning over the disc in a game. Until you're comfortable with 10th pass pressure, how can you be ready to make the throw in a real game?