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)
Other mistake: When Finn and Rey see Han talking to Ben on the bridge, they look directly down. But the platform they're standing on is to the right of the bridge. If they're looking directly down, they would be looking at either the floor or the chasm around the bridge. They should be looking to their left.
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 Rey is fixing BB8's bent antenna, at first, the antenna is shown as thin and flexible. But as the scene changes between Rey and BB8, the thin antenna changes to a thicker and slightly more rigid antenna being held between her fingers, then back to a thinner one. (00:15:53)
Chosen answer: The actors playing Rey, Finn, Poe, and Kylo Ren were mostly unknown, while Mark Hamill is famous worldwide as both a screen actor and voice actor. The billing order was likely based on their fame and then on their new roles. It's not a union thing, minimum wage in the Screen Actor's Guild is far lower than what they were paid. It's a combination of bigger star, better agent, and the future of the series.
Greg Dwyer