One source for such is Mas Ayoob referencing his experience at the NTI. Where he actually wrote it - dunno.
But I've seen it in such. At an advance tac course at Karl Rehn's - the newbies who wandered out to engage the bad guys were quite embarassed and sometimes knocked on their cans.
Now all BGs aren't going to be skilled opponents but that's the way it goes. My experience as a person in cover is that the entering team is at a significant disadvantage without overwhelming force and tactics.