It's not really a brand new thing for artists to mess up on stage. Pretty sure he rehearsed and could spit the lyrics fine. Just messed up under pressure. Although he messed up pretty bad, very bad. Dude straight bombed. But acting like it was disrespectful is kind of a reach.
Toni Braxton, Chaka Khan, Jay-Z, Adele (quite a few times on her most recent tour), and the list goes on and on have all stumbled. Even on their own songs which they've spent numerous hours working on and rehearsing. A lot of things can happen on that stage. Even some of the greatest singers have forgotten lyrics to the National Anthem at major events.
Not saying RHQ on any of their levels, those other performers have a little more luxury because of their legacies to the point where they can even just start over outright and play it off.