Well, the bit about the following week was said slightly in jest - HB did have strong opinions about what was good and what wasn't. The standard at that club - as I remember it - was pretty high, although left-field performances were often welcomed. Harry picked the singers he liked to be his residents, and gave them extended spots. Maybe that just set a standard.

"You're singing flat" may have the virtue of honesty, but I doubt how helpful it would be. The best singers in my experience are confident ones.