Something good and innocent.
Kate Herron, Loki Director, explains more here.
But I think there's something tragic, in the sense that we see her as a little girl and she's playing with her toys, and it doesn't seem like it's a big thing that she's done. So I think that's the really key thing with her is that we don't necessarily know what the exact event was, but it wasn't villainous, and it didn't seem [that way]. I think that's the clear thing, particularly with Episode 4, is that obviously our perspective on the TVA started to shift. For example, it was really important for me showing her going through the same process as Loki but it's like, 'Okay, how do we show that through the eyes of a child that's also innocent?' And we should feel, as an audience, wrongfully there.
I have my own idea of what it is, but I think in my head, it's definitely something innocent and something out of her control. Which kind of plays into the fact that not everyone arrested by the TVA is necessarily like Loki and has stolen a tesseract and created this branch. Sometimes you accidentally do just step onto the wrong leaf and you create this branch. Do those people, where it's accidental, do they deserve to go through this process where ultimately they're deleted by the TVA? Probably not. Or maybe they do, for the better of servicing and protecting the timeline.
One popular theory is that she was starting to want to become a hero like the valkyrie toy she was playing with, and so was a variant.