I'm not mad about that slip thing (which was stupid and contradicts basically every chapter where he fights/spars but oh well) but I'm mad about butlers behaviour.
It would have been ok if that guy was sibling to the noble trio, you know - young noble thinks he's better than some country bumpkin like we see in so many stories. Instead he is just a butler and holy shit, he should have been fired the moment he started shittalking guest of honour that evening. Not only that, he shittalks father of fucking S tier adventurer, equivalent to the nuke, who took down a demon lord. Which, by the way, was fucking confirmed by other people of highest caliber. No amount of "understanding" should forgive what the guy did.