I've heard it so often now that I questioned myself, wondering if I was saying it wrong all these years. I never hear "on accident" in person though. Videos, random comments around the internet I've seen it. Randoms online while gaming will say it.
When asked, AI tells me it's a common mistake. "Common." Yet, it wasn't common. Something changed. And maybe it's not common for you and your experience. Becoming quite common in mine. The ons are outnumbering the bys.