I have this very useful Compact Switching Power Supply - Selectable Output 3-12VDC from +Adafruit Industries.
I noticed a while back that the polarity was not what I thought--center-pin negative instead of center-pin positive. It didn't matter a whole lot, because I used it a lot for my 5V Regulated Power Supply, and since I used my meter to check which was + and which was -, I got away with it. Later, I found that the adapter did not work in certain applications.
I put that away for a while, but it just started to bug me again, so I Google'd the problem and was taken to the Adafruit product page (see above). Reading the not so fine print, I saw that the device can switch from center pin positive to negative. Next question: how?
I did not find an answer, so I went to look at the device. It comes with tips to accommodate various size receptacles. I turns out that the wire to the adapter has + and - on each side, and the tips have CEN with an arrow. So, if you want center pin positive, mate the side marked CEN to the side marked +, and vice versa. I never noticed that before. Live and learn. Anyway, my problem is solved. I must be some kind of genius!