So I went through the "trouble" of making a yeast starter for my latest batch (brewed on Saturday night). I pitched it at the proper temp and all that.
Well when I got home tonight there was still no activity in the airlock besides some bubbles. Then I realize I forgot to put the middle part of the airlock in. I found it, sanitized it, and tossed it on and there was immediate activity.
I'm guessing my batch isn't tainted but what *COULD* happen if you omit that part of the airlock and allow outside air to get in (even if the only air would come through the tiny holes on the air lock lid). I'm guessing enough air was being pushed out where not much could really get in - just my horribly unscientific theory.