If there's one character in the Potterverse we'd never mess with, it's McGonagall. She's tough as nails, sharp as a tack, neat as a pin and harder to get respect from than…hmm; we seem to have run out of metaphors involving pointy objects.
In fact, we think the main thing that proves Harry Potter's enduring awesomeness is that he manages to get respect from Minerva "Big Minerva On Campus" McGonagall. At least, that's what he manages to do in the original Harry Potter books.
In Harry Potter and the Cursed Child, Harry appears to do everything he can to undermine the respect he earned from her. It starts when he bullies her into spying on Albus with the Marauder's Map.
HARRY: This map will reveal to you where my son is at all times—I expect you to use it. And if I hear you don't—then I will come down on this school as hard as I can—using the full force of the Ministry—is that understood? (2.10.22)
Things are different now that Harry has authority. McGonagall, always a strict disciplinarian, can't punish Harry the way she did when he was a teenager. She has no recourse when Harry threatens her by abusing his own authority. Harry eventually apologizes, but it's galling that he doesn't show her the same respect she has always shown him.
Even under Harry's orders, McGonagall isn't a pushover. She pretends she doesn't see Albus and Severus when they hide under the Invisibility Cloak (as if that ever stopped her before). As always, she's stern, but fair.