Actually in CT you can teach .2 (the is the equivalent of one day) outside
your certification without being certified! So for instance, we have a .2
ALP (advanced learning program, gifted...etc.) position, I could teach that.
My principal would never let me, but honestly, our former teacher leader was
a reading teacher, and she taught it.
There is also some kind of thing (not sure how it works) when you get tenured
(after your 4th year in my district) that if they cut your job, you can take
the job of someone non-tenured, even if it is out of your certification.
Also there are all kinds of emergency certifications a district can allow for
a person if they desperately need someone to fill the spot.