Players with non-op permissions are able to climb into inaccessible areas by placing ghost blocks consecutively inside the spawn protection area. This allows the player to bypass spawn protection restrictions on servers if they have a certain number of blocks in the inventory, allowing to place blocks and jump on them client-side, which may be hard or trivial to do depending on latency.

This issue is only present with spawn protection in vanilla, but it's also present on certain modded servers with custom protected areas. It should be expected for this to not happen like how you're unable to clip into walls horizontally (such as rapidly opening a door or breaking slime blocks).

How to reproduce

1. Create a void world in a server with a certain spawn protection radius.
2. Give op permissions to someone else aside from you.
3. Build a tower of blocks.
4. Stay on the ground, get a stack of blocks and deop yourself.
5. Try placing the blocks and jump from them consecutively while looking at the tower (requires certain precision to achieve correctly).
6. Notice how the server validates your actions, allowing you to climb to the top.

Expected result
The player would stay on the ground, without being able to climb to the top of the tower.

Observed result
The player floats server-side by placing invisible blocks and jumping, allowing them to get to the top.