"Em" was just pondering this little gem:
I added a light fixture to an existing 20A 12awg circuit, and didn't
realize I used 14awg romex until I was done. The new fixture wouldn't
overload even a 15A circuit, so a) are you gonna tell me I have to fix my
error and put in the larger cable,

YES!

and b) why is that?

So you can sleep at night?

Thanks

Your welcome.
;-)