Continuity mistake: The little car the Grinch commandeers and crashes after going on his rampage goes from having a little flame, to fully engulfed, and back before the explosion.
Continuity mistake: When testing his engines and after he hits the wall, he turns around and is leaning on the wall. He's tilted toward the "HIT HERE" circle. The camera cuts closer in, and he's now standing straighter in relation to the circle.
Continuity mistake: After Cindy thanks the Grinch for saving her, he stops at the door. He slides his hand down the glass, and it comes off the glass well below his chin level. When he turns around, it's just at his chin height.
Answer: It was a rocket sleigh.