I was following this recipe for Chinese Stir-Fired Steak with Peppers.  I think I messed it up by cooking this for too long. The meat (I used lamb's meat) turned out very hard, like rubber.  I suspect this was because I cooked it for too long. Can someone confirm this? I will be much obliged. 
Yup, cooking most meat too long is a bad thing.  Cooking lamb too long like that will result in rubber.   Its also possible that you accidentally cut with the grain instead of against it - which would result in a very chewy, rubber like texture.  You end up with long proteins that are harder to chew.  When you cut across the grain, its a bunch of very short proteins. 