Weirdness:

• When gravel falls on top of a fence post it seems to get stuck in a falling state.
• If multiple gravel blocks fall on to the same post the entities jump up and down slightly on the post before settling!
• The player can walk through the block when it appears to have landed.
• The block is half a block lower than the grid
• Block doesn't despawn
• Relogging doesn't affect the state
• If you break the post, all the fallen blocks pop off on to the ground as items

To recreate:

Place a fence post, drop some gravel on top of it