On Agatha's discussion of queens: this got me thinking (and maybe someone else has suggested it), is it possible Lucy found the mirror in Mechanicsburg, tried to use it, and somehow got caught between for what seemed like eons to her? If Agatha's jump of two-and-a-half years was a feature of the mirrors (moving through time as well as space), Lucy could have witnessed various time periods. Freeing herself may have involved interacting with different times.

The main problems with this are that we saw her lab from the time the Other first appeared, and it looked like someone was trying to flee when whatever it was happened. That doesn't work with experimenting with a time mirror and getting trapped in it. The other problem is that we know Lu was working with mind transfer. It seems like whatever she did that made her the Other should tie into her experiments.

I was going to add that it would take colossal stupidity to try and transfer her own mind into a mirror, but we're talking sparks, so who knows? Although, if that did happen, it seems more likely she made some kind of mistake (like trying to transfer her mind under some kind of conditions that transferred her mind into the mirrors by accident).