Continuity mistake: At the start of the film, Eggman is chasing Sonic through the streets and there is a shot of the city skyline down a street behind Sonic as he makes a turn. The camera then cuts to inside Eggman's ship to a close up him with the city skyline still behind him, only now it's different buildings behind Eggman. (00:01:00)
Continuity mistake: As the Echidnas first start attacking Sonic and Longclaw, she slams the door shut quickly to catch the arrows shot at them. The In the close up of the door from the outside, the arrows are in a different pattern and are closer together than the holes when seen from inside that are a bit more spread out. (00:02:23)
Continuity mistake: Longclaw tells Sonic that if he's ever discovered, to use a Ring. Mid cut while saying this, she goes from looking straight on down at him, to suddenly her wing wrapped around him from the side pushing him towards the portal. (00:03:21)
Deliberate mistake: At the speed trap, Tom points his radar gun at a turtle. From his perspective, he seems to be looking through some sort of scope with cross hairs and numbers on it at the turtle to line up the beam. However, when he then looks at the radar gun at the speed indicated as 1 MP/H, there is nothing on the gun where he could have been looking through such a scope. At best, the top of the run appears to have plastic iron sites. The scoped view would not have been from the device itself perspective since it would not need cross hairs or numbers on said cross hairs as that would be for a person looking through it to have reference points. This scope view of the turtle was most likely just added by the film makers to have a better visual representation of Tom aiming the radar gun at the turtle. (00:04:29)
Factual error: Speed radar guns work by measuring the speed of an object that is either approaching or moving away from it. So when it shows Sonic running across, the beam giving the measurement wouldn't work, and he would have barely even registered on the device. (00:04:44)
Continuity mistake: After Sonic runs by the radar gun the first time, triggering it to show a 296, Tom picks the radar gun up and looks at it. In the view of the radar gun, he lifts it and has it tilted to the left of himself, left of screen in this shot. When it cuts to a front angle showing his face, the radar gun is now tilted to point to his right, which is now still left of screen. (00:04:49)
Continuity mistake: When Sonic cross the "tames a wild animal" for his "what to do list", the slam dunk sentence is clean again despite seeing him cross it dozens of seconds earlier. (00:44:25 - 00:44:51)
Visible crew/equipment: When Tom and Sonic pull over and the mini drone explodes, knocking Sonic unconscious, the camera pans away and you can see emergency vehicles blocking the road on the right of the screen, presumably closing down the road for filming. (00:59:55)
Continuity mistake: Robotnik's shaving his head, leaving smudges on it. A shot later his head's suddenly clean. (01:29:10)
Continuity mistake: During the chase scene with Robotnik's robotic vehicle, at one point an EMP device is launched which misses Tom and Sonic's pickup truck and instead latches onto a minivan in front of it. Except just a few shots prior, there was a wideshot of the road, and you can see that Tom's pickup truck and the robotic vehicle were the only cars on the road. The minivan just sort-of... appears.
Continuity mistake: At the beginning of the movie, we see a portion of the climax where Robotnik is chasing Sonic. At one point in the very first shot, he runs up the side of a building, onto the roof, then back down. However, when we see the complete chase at the end of the film, he never runs up that building.
Answer: First, it seems Sonic doesn't shed his quills that easily. Tom found one just by sheer luck. Second, yes that ring is likely going to play a part in the sequel.