This is a subject frequently overlooked by beginners and often to their cost. With an unbalanced plane you can get all sorts of strange flying characteristics and end up in the dirt having no idea of what happened nor of finding out, as your plane now has a totally different centre of gravity to that which it had 5 seconds ago!
This is tip stall and its not nice. It can often happen to beginners through being too timid on the throttle, especially when coming in to land. The difficulty for beginners who are teaching themselves is that you have no idea what just happened which then makes you even more timid on the throttle next time and you can guess the rest! So have look at it and try to recognise it ready for when it happens to you. A good visual demonstration is at 1m15secs on the video
More information on plane balancing. I know I keep stressing this but that's how important it is.