Wait, you did X and Y? That ticket was only supposed to be X, we have a ticket for Y in our backlog.
Re-reading the ticket for X (which was the so-called "MVP" (Minimum Viable Product)), with this new knowledge fresh in my mind, I still couldn't see where I went wrong. The ticket had decent detail, and actually references Y (the work, not the ticket) within it.
Oh, that should probably have been removed.
So, I essentially ended up doing two tickets of work in this one ticket. While I technically made a mistake, and will learn from it, I still stand by my actions based upon the definition of the ticket.
You may wonder where the problem is with this. We're now ahead, surely? Well, yes, and no. It's more complex than that.