Or I believe it was E2? Mami uses half of a Grief Seed and tosses it to Homura, who'd been watching in the shadows, and says there's one use left. Instead of using it, Homura tosses it back in refusal, and Mami seems notably agitated. They later have a confrontation scene where they essentially agree to fight if they meet again.

If I remember correctly, this confrontation is mostly egged on by Mami who's become irritated with Homura's presence in the city, as well as her treatment of Mami and her cryptic actions regarding Madoka. Then, when Homura tries to warn Mami about Charlotte, she refuses to listen altogether and just binds Homura up to get her out the way. We all know what happens next.

But what if Homura had accepted the rest of that Grief Seed from Mami? It was clearly an attempted sign of goodwill after they initially got off on the wrong foot, as let's be honest: Mami is an incredibly kind-hearted and lonely person. She would accept Homura immediately if she showed any sign of reform. After all, we've seen plenty of timelines where the three of them do work together.

Even if it wouldn't have guaranteed victory against Walpurgisnacht, it could have earned her enough respect from Mami for her to listen a little when Homura tries to warn her about Charlotte. It's just something I thought about while writing a work article about the series and wanted to share my thoughts/hear yours as well! Let's discuss about some magical girls!