After a momentary power flicker, my HP Officejet Pro gave a paper jam error. I thoroughly looked for any stuck paper and found none. I turned the thing off and on, but still got the same error. I pulled the plug from the back and put it back in and got the same error. I left the plug out over night and the next day I got the same error. So I googled the problem and one of the myriad solutions said to: Make sure the printer is turned on. Then unplug the cord from the back of the printer, THEN unplug the cord from the wall. (It is just a plain, dumb power cord as far as I can tell). Wait 15 seconds, Plug the cord back into the wall. Plug the other end of the cord back into the printer. It worked, but why? What did unplugging the cord from the wall do that unplugging it from the printer itself didn't? I have seen this work before, but I never understood why.