Error 6A00 on my Canon MP750.
The alarm light was flashing and the LCD message said to "CHECK PRINTER (PRESS [OK])"
Here's what I did (after doing a hundred other things that didn't work):
1. I opened the printer as if I was changing an ink cartridge.
2. The ink cartridge carriage did not move to the center as it should (one of the symptoms).
3. I moved the ink cartridge by hand to the far left to expose the ink purge area at the far right (under the area where the ink cartridge carriage typically parks itself).
4. I used a small make-up mirror from my wife and a flashlight to look for paper fragments or anything else that might be jammed around in the ink purge area. The mirror was indispensable for viewing areas I couldn't possibly see otherwise.
5. Eureka! I found a some ink-soaked paper fragments crumbled against a roller near the ink purge
6. Now the fun part. While wearing a head flashlight and viewing the small paper jam in the mirror, I used tweezers and needle nose pliers to extract the paper. It was like "Operation," a game I played as a kid, except every movement I made was backwards because of the mirror. It also required some moves that would challenge a circus contortionist.
7. I closed up the printer, hit the okay button, and the printer made some noise like it might work. Unfortunately, the alarm light stayed on. I opened the printer again and noticed that the ink carriage repositioned itself. I took this to be a positive sign as it didn't do this before.
8. I closed up the printer again and hit the okay button again. After several "please wait" messages on the LCD, and positive sounding noises from the printer, the alarm light went out and the printer seemed ready to print.
9. I ran a few test pages through and they printed fine.