Visible crew/equipment: When Vernita Green dies in the kitchen, during a close up, we can see a bright light reflected off the sweat on her neck. This is a crew light, as all the other light in the room is natural, coming from the window facing the other way, so it couldn't be reflected.
Continuity mistake: When The Bride and Hanzo are talking about her Japanese right before he serves her food, there is a shot of her laughing where she place her left hand on her right arm. When it cuts both hands are on the table. (00:45:50)
Other mistake: In the anime scene, after the two men empty their magazines onto the back of Matsumoto, from under the bed, O-Ren shoots the leg of the guy wearing brown, then blasts his head. Then she fires a third shot that hits a brown leg, but the guy wearing blue falls instead. (00:41:55 - 00:42:40)
Continuity mistake: When The Bride rubs off the name "Bill" written on the window condensation, the writing style is different from when it was first written by the sword maker. (00:52:45 - 00:53:30)
Continuity mistake: When the bride is holding a table, GoGo throws the mace ball at it. She then pulls the mace ball back through it. The table splits in two. The piece of table that falls at the bottom of the screen, the bride's right hand side, falls completely apart. The legs have broken away from the table top. In the next shot this piece of table is standing on end with the legs reattached. (01:18:55)
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)
Continuity mistake: When The Bride cuts Sophie's arm off you see her get covered with blood. In the next shot, however, there is considerably less blood on her top and it is in a different pattern. (01:11:20)
Revealing mistake: When the Bride takes a slash at Buck's heel, you can plainly see that there is a fake heel there. She definitely cuts through the sock, so it's not the sock pattern, and there is no blood.
Continuity mistake: When O-Ren takes off her sandals to fight the bride she pushes her right sandal out from under her. It slides on top of the snow and does not push around a lot of snow. No snow collects around it. In the next shot when she takes off her left sandal, the right sandal now has a lot of snow that was pushed up around the side of the sandal and collected beside it. It is also sunk much deeper into the snow than in the previous shot. (01:31:35)
Continuity mistake: When O-Ren Ishi tells 'Charlie Brown' to get lost we see some blood stains on The Bride's face. The stains differ in the next few shots of her, then later when we get a close-up of her face the blood stains are the same as earlier. (01:12:00)
Continuity mistake: When Boss Tanaka breaks the bowl on the table, a piece of meatball rolls on the table. Later when the other Japanese guy throws a hand towel on Tanaka the meatball is gone. (00:59:00 - 00:59:45)
Continuity mistake: As Earl McGraw tells his Son Number One the massacre at Two Pines house of worship must have been "the work of a salty dog," he gingerly moves in for a closer look at the carnage. Even though the side shot maintains a close view on the upper half of the body, the posture of his movements indicates that he starts stepping forward with his right leg. Yet in the immediate next shot from behind, it is with his left leg that he makes his advance. (00:18:30)
Continuity mistake: When O-Ren's young bodyguard has the mace, she hits the Bride smack in the chest more than once with the very heavy, spiked ball. But later, there is not a trace of a welt, bruise, or cut on the Bride's chest (plenty of blood on her half-zipped jacket but her skin is just fine.)
Factual error: In the beginning of the anime sequence we see O-Ren's father from behind, along with his Army rank on his left shoulder. The rank in the movie has two chevrons on top (the upside down "V"s), three "rockers" on the bottom (the bowl shapes) and a star in the middle. The real Army rank of Sergeant Major has three chevrons on top - no rank except corporal has only two chevrons, and corporals have no rockers or other insignia. (00:35:15)
Continuity mistake: At the beginning of the fight between Oren and the Bride in the garden, Oren puts the sword up to eye level and starts to withdraw the blade from the scabbard. The shot switches to a view from behind her and we see the back of her head. The eye-level sword, however, is nowhere to be seen. When the shot switches back to Oren's face, the sword has reappeared. (01:32:20)
Factual error: Near the end of the anime sequence when O-Ren murders the crime boss, there's a shot of the wall/ceiling behind her as she pulls the sword out of his stomach and you see the outline of O-Ren in the blood on the wall. In the next shot of O-Ren's face and body she only has minimal amounts of blood on her. Considering there was enough blood spraying out that you can see her outline on the wall behind her, shouldn't the front of her be covered in the blood as well? (00:40:05)
Revealing mistake: During the restaurant fight scene, Thurman's fight double can be easily spotted from time to time as she is a few inches shorter and a little heavier than the star. It is especially noticable during the silhouette sequece. (01:23:30)
Factual error: When lying in the hospital, The Bride's heartbeat (the "beep") doesn't match 69 (per minute) which is on the monitor and also doesn't match the visible flow in the artery in her neck (a few seconds later). (00:24:50)
Continuity mistake: In the scene where The Bride wakes up, a mosquito is shown landing on her. The first shot of the mosquito it shows it landing with all legs. Then in the close-up shot sequence, you see the mosquito is missing a front left leg. (00:24:20)
Continuity mistake: Oren is in the back of a limo, with all six motorbikes ahead of her, after the final two have moved up from the back to the front, but when we see a shot of Oren, in the limo, there are four headlights behind the limo, the way it was before the bikes moved up.
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.