You can actually see through the speeder bikes and their pilots, and see the forest scene behind them. This is very similar to the snow speeder slip-ups from Star Wars Episode 5: The Empire Strikes Back. This is best seen during the scene where Luke jumps from Leia's speeder to an Imperial speeder. It is also quite noticeable during the shots leading up to that when we have a high rear view of the back of the speeder bikes.