I'm going through the sprite fighting scenes at x0.25 speed, and only have just noticed that the Felt members that look to the left have their numbers on the cap mirrored. Seems like Hussie simply mirror flipped the original sprites.
Not very surprising, since these scenes contain SO MUCH ACTION and he have had to do about 70% of that animation in just two months, it would be expected that he would try to cut some corners, but still.
EDIT: also, at about 4:56, Jake seems to partially turn into a Twilight vampire.
EDIT2: 5:07, Dirk points at Jacks and then gives a thumbs down, while his glasses glitter with reflected light.
How are there so many details this is ridiculous
EDIT3: There seems to be some kind of discontinuity error at about 5:08. Jack and Spade have their weapons swapped for some reason. More like they should be swapped themselves.
EDIT4: 5:16 - you can partially see Spades Slick hat lying on the ground to the left.
At 6:16, Ms. Paint seems to have changed clothes during style transition.
At 7:35, Hussie's Character Aura seems to have the following characters "crossed out" (not in any particular order): Terezi, Jane, Dirk, Eridan, Vriska, Nepeta and Karkat. Also what appears to be Dave, almost out of frame. Wonder what that could mean.
Also Hussie appears to have some pretty weird "is punched out and now is rotating uncontrollably" sprites.
EDIT5: Found another small glitch: at 12:53, the background for the meteor is reddish, in contrast with the rest of the image, which has black one.