Can I back out on closing day itself if I notice something new during a final walkthrough?

It depends on what you actually found and whether it ties to something still open in the deal. If the new issue represents a real breach of the seller's representations, or means a closing condition still isn't satisfied, you may have a legitimate basis to delay or refuse to close. If it's more like ordinary wear, something already reflected in the disclosure schedule, or a risk you already accepted when you signed the purchase agreement, backing out on that basis can itself be treated as a breach on your part.

The stakes of getting this wrong run in both directions: closing anyway when there was a real, unresolved problem can make it much harder to raise later, while refusing to close without a solid legal basis can expose you to the seller's own claims, including loss of your deposit. This is exactly the kind of decision to flag to your lawyer immediately rather than making the call yourself in the moment — a quick call before closing proceeds is far better than trying to unwind a decision after the fact.