Going by what's in the extant nodeballs, the following node IDs never had any node, at least on PM: 17, 38, 103, 104, 105, 108, ...

It appears that gaps naturally arose as a result of loading nodeballs into the database. They would typically be loaded to occupy a sequence beginning at some number "safely" above the current highest node ID.

What's funny is when that would happen, and then some user-generated node would get created, such as a user, and then another nodeball would get loaded, above some small gap.