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Let them learn It for themselves
This is not just an IT issue, but an issue that applies to most fields. Unless the situation is a matter of life and death(ex. a bridge in Minnesota), it has been my experience, that challenging your boss, is a battle best left not fought. Your boss needs to see the failure 1st hand, and be able to realize that they had a part in it. Then and maybe then, they will ask other people for their input, and actually listen to them. Most of the time this means you are in a Lose-Lose situation.