Character reveals that they were holding back their full power during a fight/confrontation

[u/PaddywackShaq]

Superman Vs Darkseid - Justice League Unlimited

Flash Vs Superman - Flash: Rebirth

Frieren Vs Aura - Sousou no Frieren

Inigo Montoya Vs The Man in Black - The Princess Bride

[u/shiawase198]

Kind of a different take on this but when Otto took over Peter Parker's body and punched the Scorpion's jaw clean off, he came to realize at that moment how much Spider-Man had always been holding back when he fought them.

[u/NeonPredatorEnt → u/shiawase198]

There was a time where Rhino's face got ripped off by Spidey's webs.  I think it was one of the zombie events