Hello! As an animator, I’ve been encountering errors for some time now, and I suppose many others have too. One of them is body parts making odd and random movements (often sporadic/spazzing). I have found the way to fix that, at least in my experience. It’s a simple fix; once you figure it out, you’ll get a red face from a facepalm (and maybe also collapsing on your keyboard in that moment of “How did I not notice?”) Look at this video. As you can see, the animation plays perfectly in editor mode.
But if you go in-game or play the animation, you get something like this:
Notice that when you play the animation (even in-game), the glitch happens, but if you go back to the editor and swipe through the timeline, it looks normal; however, if you look at the part of the timeline showing which body parts are moving in the keyframe, sometimes, a body part may be missing from a keyframe. Look at the below picture. The arrows indicate that, even though the arm has changed its position, it isn’t listed on the timeline.
To fix this, go to the keyframe with the red marker and select the body part that is missing from the timeline even though it is moving. Next, choose a controllable increment if you haven’t already (I recommend the defaults or higher) so that you don’t accidentally move the body part. You then click the handle of the rotation tool or arrow of the move tool (don’t move it unless you want to change the animation for whatever reason).
Check to see if the body part is now on the timeline. Test to see if the animation works now!
Hope this helped! 
