Just discovered something while building my LM7805 supply. I have two wall wart (AC adaptors)
a) Stancor(12VDC, 200 mA) and b)CUI SMPS 12V 1.5A.
a) has +----(0----(-) on the barrel connector and
b) has (-)---(0-----+ on it.
I assumed both to be the same and plugged them in. Luckily, I had my protecting diode. I wish they had the same convention for all wall warts..Anyway thought I would share this with the community. :)