I have been inside the E/E compartment of the 777 during flight. We had to push back a few CB's which were not pushed back during a hangar visit. (all lavatories didn't fuction) we found out about 30 minutes after departure when the first passengers started going to the lavatories.
Took a portable O2 bottle with me, in case of decompression while I was down below. took about 3 minutes total time. Saved us from having to turn back.