I just finished my first quad build (450,APM 2.6). It started off ok and then shot straight up to about 20". I could not shut it down with my throttle. It stayed in one spot until it drifted into a palm tree and disconnected my battery. Luckily no damage. I tested the motors afterward and they shut off fine. Is there any setting I should look at and may have missed? This was not a good feeling. 
Can you please post a log? It sounds like you were in a failsafe mode and it was returning to home.
zlite, I will try to post a log. I need to figure out how to do that first. I will search the APM site on log files unless you can tell me quicker. Thanks.