Two members of a team (mercenaries, crime fighters, knights errant, what have you) regularly practice sparring together, and the intensity of their bouts starts to escalate. The rest of the team watches in concern, but hey, they both agreed to it. It’s a little weird, but there you are. Eventually, the two are absolutely beating the crap out of each other every day.
Teammate A is enjoying themself, because they’re a sadomasochist and this is a great (platonic) way to enjoy hurting someone and getting hurt while also improving their skills. They assume it’s the same for their teammate, who often initiates their bouts, and are glad to have met someone else with the same interest.
In fact, Teammate B is punishing themself for something, perhaps a failure during a previous battle, and they assume Teammate A is so angry at them, hates them so much, that they overlook their own injuries in favour of hurting Teammate B. Clearly the rest of the team supports this as well, or they would have intervened.
There’s potential for further emotional whump here when Teammate A eventually learns that what they thought was a cheeky bit of S&M was actually a way for their friend to hurt themself. They were (unintentionally) being a whumper because they didn’t bother to negotiate their gd kink.
(Extras: Does Teammate B forgive Teammate A? Do they even blame them at all, or place all the responsibility for the misunderstanding on themself? What does the rest of the team think, other than that their teammates are both tragically dense?)