Continuity mistake: After Rey touches Luke's lightsaber she begins to have the vision, and when she falls to the floor the blaster is missing from the back of her belt, but after she "sees" Luke and R2 and then stands up the blaster is back again, tucked into her belt (even though Rey's having her vision of things all around her, she herself is not a vision).
Continuity mistake: After Rey's lightsaber battle with Kylo Ren, when the Falcon appears in the wideshot Rey's head is turned to look at the Falcon behind her, to her left, with Finn to her right, but in Rey's two closeups - before and after that wideshot, she's looking straight ahead at the Falcon, with Finn lying directly in front of her. (01:58:50)
Continuity mistake: After Leia tells Han that she always hated watching him leave and he still drives her crazy, they hug and it cuts to the Starkiller Base. When we see Stormtroopers walking through the corridor and then Rey sneaking away, it's a flipped shot - note the Stormtroopers' backs, and Rey's hip bag and leather wrist cuff. (01:35:15)
Continuity mistake: When Rey wakes up restrained as Kylo Ren's "guest" at Starkiller Base, Kylo Ren's mask has distinctive surface markings and a hole, but when he removes his mask and sets it down in the closeup it's a flipped shot - the marks and hole have switched sides. (01:25:50 - 01:26:25)
Continuity mistake: After Rey finishes her meal beside the foot of the AT-AT and puts on the Rebel flight helmet, there's an overhead wide shot of the toppled AT-AT and we can see its front knee joint. When Rey hears the commotion with BB-8 she rushes to get her staff, which is now tall and upright in the sand at that knee joint, and had her staff been there in the wide shot we would have seen it. (00:14:45)
Continuity mistake: When Snoke orders, "If what you say about this girl is true, bring her to me," Kylo Ren is wearing his hooded cape. It cuts to Rey restrained in the holding cell, and when it cuts back to Kylo Ren rushing to the holding cell to get Rey and bring her to Snoke his cape is gone. After Rey uses the Force to trick the guard, it cuts back to Kylo Ren arriving at the cell and he's once again wearing his hooded cape. (01:30:40 - 01:31:20)
Continuity mistake: When Rey rescues BB-8 from the scavenger Teedo, the top part of Rey's outfit is crisscrossed with her right side over left, but when she shushes BB-8 it's crisscrossed with her left side over the right. Then it's back to the original way. (00:15:25)
Continuity mistake: When Poe and Finn steal the TIE/SF, both of Finn's torso harness straps have thin pieces of ridged plastic attached to them - nothing else, but when Finn wakes up on Jakku those pieces attached to the straps are entirely different. (00:20:45 - 00:24:00)
Continuity mistake: While Finn, Rey, and BB-8 are hiding in the maintenance bay to unfix the poisonous gas, when Finn lifts the floor's slotted deck plate and pokes his head out there are specific grooves at the edge on top of the plate, but no grooves underneath at the edge. The problem occurs when Chewie lifts the deck plate, and those grooves should be on the side facing him (the top side), but instead they're facing out. Then when Han tells Finn "that thing" can understand him, the grooves are gone - the top side of the deck plate is properly facing out. (00:40:05)
Continuity mistake: When Kylo Ren interrogates Rey, he removes his helmet and sets it down on a table of ashes. This table is not present in any of the wider shots of the interrogation room. [Director JJ Abrams has stated that the closeup of the ashes (the ashes of his victims, apparently) was actually from an earlier scene taking place in Kylo Ren's quarters on the star destroyer, which explains why the table is not visible in the Starkiller Base interrogation room. Still a mistake but there's the reason.] (01:26:25)
Continuity mistake: After the First Order has destroyed the Hosnian system, including Hosnian Prime, Finn rushes up to Han and asks him where Rey is, and when it cuts to the shot of Rey running in the forest it's a flipped shot - note the leather cuff is on Rey's right wrist, etc., then in the next shot the cuff is on her left wrist as it should be. (01:11:05)
Continuity mistake: When Rey fixes the Falcon's electrical overload she pulls out blue and red wires, but just as she says, "I bypassed the compressor," in this shot facing her there are only four blue wires - no red, and then the red wires reappear with the blue. (00:51:55)
Continuity mistake: After the Quad Jumper blows up, when Rey, Finn, and BB-8 run to the "garbage" they're running quickly toward the Falcon's forward side - note the cockpit, with its entry ramp lowered and nothing blocking its way, but in the next closeup they're all running from the rear of the Falcon before they rush up its entry ramp. (00:31:45)
Continuity mistake: During Rey's lightsaber duel with Kylo Ren, a few shots after Rey does the flip and slices the tree branch, there's a long shot of them in the forest as they're both moving past very distinctive fallen trees around them, and after a few closeups as Kylo keeps advancing on Rey, at the start of the next long shot (right before the chasm first appears behind Rey) they're back to being where they were in the previous long shot. (01:55:50)
Continuity mistake: When Rey reaches the top of the ridge and shouts at Teedo, we see there are no footprints in the sand between her and Teedo, so when Rey rushes to BB-8 her lone set of footprints are the only ones behind her in the sand. When Rey walks away from BB-8 telling him "No," there are many more footprints in the sand between her and the droid, even though there should only be the two sets of Rey's footprints - from both coming and going. (00:15:10 - 00:16:05)
Continuity mistake: When Rey returns to her home in the AT-AT she scratches another mark into the wall, and in the next closeup just as she sits down the top part of Rey's outfit is crisscrossed with her left side over the right and her hip bag is at her left side, but after Rey fills her plate and takes her food, she stands up and her top is crisscrossed the opposite way, etc. The first shot is flipped - note the background, compared to the second shot. (00:13:55)
Continuity mistake: After the TIE fighter crashes on Jakku, when Finn searches for Poe in the wreckage he finds Poe's jacket and grabs hold of the jacket arm, but when it cuts to the following shots as the ground starts shaking, Finn's holding the top (neck area) of the jacket, even though it never left his hand. (00:24:35)
Continuity mistake: After BB-8 tells Rey that the Resistance base is located in the Illenium system, while Rey is down in the hold she asks Finn for something she needs and she holds her right arm up and points, but in the shot facing Finn it's her left arm that's up pointing as she repeatedly shouts "NO!" to him, and then it's her right arm up again. (00:38:40)
Continuity mistake: After the Starkiller Base weapon is fired at the Hosnian system, when it cuts to Finn loading gear onto Ithano's ship he stops at the top of the stairs and turns to watch the sky. The blaster's strap is beside Finn's neck slung crossbody, but in the shot from behind there's no strap near his neck. (01:10:20)
Continuity mistake: After the spy, Bazine, communicates that she's found the droid at Maz Kanata's, in the following side closeup of Kylo Ren we have a good view of the distinctive surface dents/hole on the side of his mask (his left side) and this is a flipped shot, then in the front closeup of Ren those dents/hole are now on the opposite side of his mask with different marks the other side. (00:58:55)
Answer: Any answer would be conjecture - best just watch the next movie.