OK so it was using the blueline contoller and either that failed, the temp sensor part of it failed, or the temp sensor got knocked out of the water causing the heater to stay on.
As far as I know that blueline unit should be fairly high quality (not positive, but in general they make good stuff). What I think most of the people were referring to when we were talking about controllers was aquarium system controllers like the reef keeper, apex, Reef angel exct. Idea being if you have two controls then either can save you in the case of a run away heater. Ie, use the built in thermostat, or the heater controller like the blue line and then have that plugged onto the system controller with one temp setting set just a hair higher than the other so that if one fails and sticks on the other will save the tank.
*Sorry if I just re-stated the obvious. Sometimes it's hard to tell if everyone is following the discussion or not??
Is it possible that the blueline controller's temp sensor got knocked out of place? That seems like the easiest possible explanation for what happened.