Christ, there's a few things in the above posts alone I can concur with and comment on. I guess if I'm only allowed one mistake per post then I'll go with . . .
This. It took me probably longer than it should have to realize that I can't cruise through the warmup weights on autopilot, regardless of how light they feel or how easily I can power/muscle them up. Every time you wrap your hands around the bar you are practicing technique, reinforcing movement patterns, finding the mental zone. You should be seeking perfection with every rep. This is something I still struggle with sometimes depending on my mental state.