Yeah, now that you mention it I never got around to limiting how far you could go: the "Player" object is really just that particle effect and apparently it's not affected by the obstacles I put in for the other characters. By the time I realised that was likely to be a problem, it was too late to fix it. With more time, I think having the camera zoom out to include all zombies on screen might be the way to go.
By the way, possibly a weird place to mention this but I read through a whole lot of Housepets (I think the whole thing at the time) back when I headed off to university. I didn't realise you were into game jams and whatnot as well!