Continuity mistake: At the Resistance base, when the doctor's treating Chewie's injury, she's using a medical device with her right hand as she's telling him that he must be so brave, but when it cuts to the next shot it's her left hand up using the device. (01:22:55)
Continuity mistake: After the TIE fighter crashes on Jakku, the closeups of Finn waking up and releasing himself from the seat strap while he's lying on the ground are flipped shots. (00:24:05)
Continuity mistake: After Kylo Ren uses the Force to lift Rey and slam her into a tree Finn kneels beside her. When Ren calls Finn a traitor, Finn turns to his right and starts to stand up, but in the next shot Finn has stood up by turning to his left. (01:51:40)
Continuity mistake: After Rey scrubs the salvaged parts clean, when it cuts to Rey standing in front of Unkar Plutt her shoulders are quite a bit taller than the shelf in the wideshot, but in the closeup the shelf is at Rey's chin level. (00:13:15)
Continuity mistake: While Rey's fixing the motivator, when Finn asks her if she wants to go back to Jakku because of family or a cute boyfriend, Finn's kneeling on his hands and knees, but in the next shot he's not, just as Rey tells him it's none of his business. (00:39:00)
Continuity mistake: When Rey unloads the net filled with parts at Niima Outpost, as she picks up her staff to her right we see the black bag hanging on the side of the speeder (note the bag's straps are now much shorter than when she drove off on the speeder), but in the next shot as she continues to pick up her staff, which is now to her left, the black bag is suddenly gone from the speeder (when Rey pulls the net it looks as if the black bag is lying in the net with the parts, though she didn't put it in there), and the net is entirely unattached as well. (00:12:40)
Continuity mistake: After getting up from his crashed ejection seat, when Finn sees the smoke and runs toward it his shadow's directly behind him. Just as he reaches the crashed TIE fighter his shadow's in front of him to his left, but when he faces the front of the TIE fighter as it sinks into the sand, Finn's long shadow is still being cast in front of him, even though he's now facing the opposite way, then a thin long shadow is cast onto the sand in yet another direction, to the left of the exploding TIE fighter. (00:24:20)
Continuity mistake: During the exchange between Ray and Solo note the compressor she is holding. The wires are at the top, next shot these are at the bottom, then they are back at the top, and in the last shot the wires are gone. (00:51:55)
Continuity mistake: At the Resistance base, after it's said that, "The weapon will fire in two minutes," it cuts to Rey and Finn running in the woods on the surface of Starkiller Base, and the leather cuff is now on Rey's right wrist, presumably due to it being a flipped shot. (01:50:35)
Continuity mistake: When Finn asks Rey if she can unfix the poisonous gas they rush to get the air masks, and as Finn climbs down to the maintenance bay below deck we see the floor behind Rey when she starts to kneel to help BB-8. Then after the closeup of Rey helping BB-8, in the next shot as Finn takes BB-8 from Rey we see the corner of the floor's deck plate that has suddenly appeared behind Rey, which she reaches for in the following shot to cover the maintenance bay. (00:39:45)
Continuity mistake: During the escape from Jakku, the Millennium Falcon is battling two tie fighters and Finn's cannon gets stuck in the forward position. After Finn shoots down the 2nd tie fighter, he is shown celebrating. In the shot, Finn's cannon is no longer stuck in the forward position and is now pointing up. (00:35:35)
Continuity mistake: In the scene right after Rey flies the Falcon inside the crashed ship, she cuts the engines and the Falcon begins to rotate nose down, flipping over so that Finn can shoot the TIE fighter down. We see the Falcon falling upside down, then it cuts to Rey slamming the throttle forwards. When it cuts back to the Falcon, it is falling nose first and pulls up, narrowly missing hitting the sand, but is right side up. (00:34:15)
Continuity mistake: When the imperial officer is informing Kylo Ren that they were unable to find the droid, Kylo slowly turns his head around. In the next shot, from far away, red lighting has appeared on the left side of his mask. The shot cuts back to him and the red lighting on his mask has vanished. (00:36:50)
Continuity mistake: When the members of the Kanjiklub arrive on Han's ship, the airlock opens, showing two members standing ahead of three others, with a gap between the two in front. The shot cuts to a closer shot and the distance between the two has decreased. (00:44:35)
Continuity mistake: When Kylo Ren and Han Solo meet on the bridge in Solo's final scene, they both stop directly next to a red bridge light. As the shots change throughout the scene, Ren is shown standing even with the red light, behind the red light, and in front of the red light. Solo's position in relation to the red light near him changes several times as well. (01:46:50)
Continuity mistake: When the general is giving his Starkiller speech, there is a Stormtrooper is on the far left of the screen with a red shoulder pad which is down. It is fixed later in the sequence. (01:08:40 - 01:09:15)
Continuity mistake: When Finn retrieves the jacket from the downed TIE fighter he's standing right in front of it, but when it starts to sink he takes a couple of steps back, then the long shot shows him further away, and even further when it explodes. (00:24:40)
Continuity mistake: In the scene on the bridge when Kylo Ren kills Han Solo, if you look carefully at Kylo's lightsaber during the series of shots immediately before he ignites it, you'll see that the saber is pointed completely the wrong direction - if Kylo ignited the saber the way it's pointed in those shots, the blade would come out perpendicular to the direction it would need to come out to kill Han. Then Kylo ignites the lightsaber, and suddenly it's pointed the right way, even though he never rotated it. And yes, Kylo's lightsaber is a crossguard, but it's the main blade that kills Han, so this is definitely a mistake.
Continuity mistake: When the falcon is caught in the tractor beam, in the shot after Finn says "it's the First Order," the shot cuts to Rey saying "What do we do? There must be something." There is a strand of hair on the right side of her head, and as the shot ends, the tip of the strand is pointing away downwards. In the next shot, the tip of the strand is suddenly curled up and closer to her neck. (00:39:35)
Continuity mistake: When we first meet Rey, she is scavenging Imperial wreckage, and she slides down the sand dune on a makeshift sled (she left her speeder at the base of the sand dune, which means she trudged up that massive dune). When she reaches the bottom of the dune, her speeder is casting a distinct, dark shadow on the sand. As she fires it up and rides away, the speeder suddenly casts no distinct shadow on the sand, even after she reaches her destination and parks it. (00:12:00)
Answer: Any answer would be conjecture - best just watch the next movie.