Could be things like bad habits or you didn't listen to instructions. Good question though. They could also have failed for medical reasons (Short legs, ECG etc.)
Did you look read <a href="http://forums.army.ca/forums/threads/36730.0.html"> this thread </a> ?
Look at "Spidy's" post.
First off, I'm a Griffon pilot who's been flying operationally for 3 yrs now, did ACS in 1994.
You guys are getting too wound up over ACS. It doesn't test your flying abilities, it tests your abilities to learn and follow instructions. Flying 20' low? You're not following instructions. Flying 100' high? Not following instructions.
Go in there with an open mind with a good attitude. There was a commercial pilot on my ACS that failed. Me? 0 flying experience whatsoever. I did fine. You guys should stop worrying about what pedals to buy for your computers, and start planning just in case you fail ACS. Don't centre your life around becoming a pilot, because ACS is by far the easiest step in the process to get military wings.
There are only 2 reasons for failing ACS - you couldn't follow instructions accurately, or you weren't medically fit. Simple as that. Sorry to be so, um, brutally honest, but the staff there aren't there to help you pass. That part is up to you, and there isn't enough MSflight sim time in the world that will help you.
There is also <a href="http://forums.army.ca/forums/threads/36026.0.html"> this post</a>.