Making those duh mistakes is always a good sign to walk away. For me, it's when my arms feel too heavy to lift, my shoulders are so tight they are up around my ears, and/or a sudden neck spasm pulls my head sideways. LOL, Actually, those last are signs I should have quit a while ago. Mainly, when I feel my shoulders so tense and I can't relax them down with biofeedback, then I know I need a break before it gets any worse.