One possible cause might just be that some cleaning fluid made it's way through a cylinder on the same side as the questionable HEGO and landed on the heated element - cracking it.
The sensing element is a ceramic matrix if I'm not mistaken and once hot it doesn't like cold stuff hitting it. The thermal stress is what cracks/kills the HEGO.
I had both of my upstream HEGOs quit within a week (with intermittent faults thrown in the mean time) of being stored in a cold damp garage last winter. When I started the car to pull it out and go home it stuttered and stalled then ran pig-rich for ~ 2 minutes before clearing out.
A question for you would be - did you leave the key on for about 20-30 seconds before trying to start? That's about how long it takes a HEGO to heat up.
J
