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: This error only occurs in the Japan version: Here, the little boy (the only one who doesn't wear a mask) "The Bride" fights before confronting the head general of the Crazy 88 had already fought against her in the middle of the battle with the rest of the gang. In that short fight she cut his mask off (explanation why he doesn't wear it at the end), realised he was only a child and then instead of killing him threw him into one of the pools. You actually see him falling into the water. But at the end when they meet again, he is completely dry (except for his sweat). (01:28:35)
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 O-ren chops of Boss Tanaka's head, the head is going in the direction of the boss to the left of Boss Tanaka. It is already on its way down and at eye level before the shot changes. The next shot we see the reaction of that boss and the other bosses but the head is no where in the shot. The head then reappears in the next shot and lands in front of him. (00:02:55)
Revealing mistake: When The Bride is calling out O-Ren, Sofie Fatale has her arm on the building's beam. If you look at her arm, right where The Bride is about to cut, there is a visible seam in the sleeve. (01:11:10)
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: During O-Ren's speech the blood on her forehead changes between shots.
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: When O-Ren Ishii beheads Boss Tanaka, the bald boss sitting next to Tanaka alternates between holding a fan and not. After Tanaka's head is chopped off, there's a quick shot to bald boss to see his reaction, and he's holding a fan. When the shot goes back to him a second time, the fan is gone.
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: During the restaurant fight scene to get O-Ren, the blood on The Bride's face (particularly her nose) keeps changing. This may be an homage to older martial arts films, but at one point towards the end it is in the same spots as it was in the beginning as well.
Continuity mistake: When Gogo gets hit by her metal ball, she falls and breaks a table, and a lot of splinters gather around her head. In the next angle, for only a couple of frames, there are fewer splinters.
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 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)
Deliberate mistake: When the Crazy 88 crew is coming in to attack the bride they use one of the shots twice. It is the one that shows 1 guy jump over a banister and go to the left of the screen then another guy comes directly after him and heads directly towards the camera. It happens at 1:21:49 and 1:21:53. (01:21:45)
Continuity mistake: O-Ren shot the dart from where she was sitting in the private dining room at the restaurant. Since we see the dart travelling in a straight line past the kneeling Bride's face, the dart somehow manages to stop at eye level when Go-Go retrieves it.
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: When Hattori Hanzo gives The Bride her sword, his assistant is sitting behind him in a cross-legged position. But the direction the assistant is facing keeps changing. (00:55:40)
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.
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.