Dead keys intriguing

Bought a 2011 MBP 15 for cheap because the right Shift and Return keys didn't work. I plan to replace the keyboard eventually (happy to use apple bluetooth for now) until just now I accidentally went for the '?' (right shift+/) and it performed the function of F6 (keyboard backlight brighten). 'left shift+/' makes a '?' as it should. What's up?

edit: I should note that I wiped the HDD and CCC'd my 13" drive onto the disk. shift/return keys didn't work before or after on the 15"

 

External apple bluetooth keyboard operates normally. i.e., the "r_shift+/" gives the normal '?' and "Return" key works properly.

As stated in edit, I erased PO's OS and reinstalled with Lion Recovery Partition. Also ran AHT, both the quick and the long, and it said there were no problems. I specifically asked if the PO has spilled liquid on the keyboard, stating that I would be checking the moisture indicators, and he said that had never happened. Based on his word, they just stopped working one day.