Revealing mistake: When the swordmaker writes "Bill" on the window, take a close look. The window has been wiped before (as happens at the end of the scene) and "re-steamed" - you can see the outlines of the wiping from earlier takes, with a clear contrast between that pane and the others. (00:52:45)
Continuity mistake: When Vernita and The Bride are talking in the kitchen and Vernita shoots The Bride from the cereal box, you can see the bullet hole in the back wall of the kitchen, but then a little later that part of the wall doesn't have any bullet holes. Also, not only is the bullet hole not where you see it hit in the first place, it's on the other side of the bride's head, along with the splatter from the cereal box.
Continuity mistake: When Vernita Green takes her daughter's picture from the notice board in the kitchen she holds it at the top left corner when the camera is behind her. The shot changes to in front of her and she holds it by the bottom left corner, the shot moves behind her again and she holds it by the top left corner. (00:11:15)
Continuity mistake: When "The Bride" is first in Okinawa and talking in the bar to the sword maker, look at his headband. The black and white stripes change from shot to shot, obviously showing how each time the material was twisted by the costumier before shooting. (00:44:15)
Continuity mistake: When O-Ren falls dead in the snow, only the snow by her feet is pushed up. In the next overhead shot there are big footprints/marks in the snow near her legs, and her samurai sword has also moved. (01:33:35)
Continuity mistake: As The Bride writes her "Death List Five" on the plane, the handwriting of everything greatly changes between shots. (01:38:15)
Continuity mistake: When Elle Driver carries the tray with the syringe on it, to The Bride's room, the syringe needle is exposed. But as Elle is preparing the injection, she removes a cap from the syringe. (00:20:35)
Continuity mistake: When Hatori Hanzo is talking to the bride, he calls for his assistant several times. At one point, he gets really mad and throws his knife unto the magnetic strips on the wall behind him and sticks on almost vertically. In the next few shots, the knife is positioned in different angles. (00:47:50 - 00:49:10)
Continuity mistake: When The Bride and Vernita are chatting in the kitchen after their knife fight, the sweat on The Bride's shirt changes. It is a small portion at the neckline of the shirt to covering most of the front of the shirt, to back to a smaller portion. (00:11:00)
Continuity mistake: In the fight scene with Vernita Green when she and the Bride are staring each other down as the school bus pulls up Vernita is holding the knife horizontally with her index finger along the top. The angle of the scene changes and she is holding the knife in the opposite way. When the scene changes back her finger is along the top again. (00:07:10)
Continuity mistake: When Vernita Green and the Bride are talking to Nikki, Vernita has no blood on her right breast, but when they are talking in the kitchen, there is suddenly blood. (00:08:10 - 00:09:55)
Continuity mistake: After Black Mamba leaves Vernita's house, she looks at the "Death List Five" notebook page and crosses off Vernita's name. The handwriting on this page (sloppy) differs greatly from that of the list written on the plane (neat) after killing O-Ren Ishii, which is supposed to be the original list. Even if she re-wrote the list, the handwriting wouldn't have changed THAT much. It is a different person's handwriting and not just a difference between sanity and madness. (00:15:40 - 01:41:50)
Continuity mistake: The Bride kills Vernita in the kitchen then pulls out the knife. When Vernita's daughter enters the kitchen there is a front shot of The Bride holding the knife behind her, hiding it from the child. The camera switches to a reverse angle and The Bride is holding the knife in front of her, in full view of the child. (00:13:30)
Continuity mistake: When Black Mamba goes into the house in Pasadena, California, to the bottom left of the slide is a ball which is not there when she leaves. (00:05:25 - 00:15:15)
Continuity mistake: During the fight at Venita's house, she overturns a large shelving unit onto the Bride. A shot from the floor (behind the Bride's head) shows nothing on the floor to the right. As the shelving begins to fall, a basket of magazines suddenly appears and the left edge of the unit is seen falling to the outside of the basket. A moment later Venita bolts for the kitchen and the shot cuts back to the Bride. The basket is now outside the edge of the unit (and some of the shelves themselves have disappeared as well). (00:06:40)
Continuity mistake: When The Bride and Vernita fight and during their kitchen chat, Vernita's zipper is in the black-band portion of her sweater. After the knife penetrates Vernita, the zipper is now higher up into the white-band portion of the sweater just above the knife. (00:12:30)
Continuity mistake: During the "The Man from Okinawa" chapter, The Bride picks up a sword from one of the racks on the wall. She slides the sword out from its scabbard approx. 4 inches and stops. The next shot shows the reflection of her eyes in the blade. In the third shot, we see a wider view of The Bride again but now the scabbard is much farther out than where it used to be. (00:50:25)
Continuity mistake: When Uma Thurman is standing in the kitchen talking to Nikki, the position of the strap of her knife holster rides up and down her thigh between shots.
Continuity mistake: When The Bride is wheeling along in the wheelchair out from the elvator, as she passes the corner into the parking lot, you can see a black car, a white car, a green car and a big red jeep parked in a row. Then when she passes the cars, the red jeep is gone. (00:31:05)
Answer: From what I understand the reason for the lights being off is the same reason for why the previous scene was done in black and white; to decrease the amount of 'graphic violence' in the movie in an attempt to keep an 'R' rating. I would assume that they had him shut off the lights for that scene as just another method to accomplish that task.
I believe the original question was asking why was it done within the context of the film (i.e. why did the character shut off the lights) not why was it done in reality. My best guess is that the manager switched off the lights thinking the 88 had a better chance of killing the bride if she couldn't see. True, they couldn't see either but there were so many of them one could possibly have gotten to her.