Projectiles collide at different areas on glass panes, fences, cauldrons, composters, brewing stands, iron bars, and fence gates than those blocks' bounding boxes are actually located at for other entities. The bounding box is ignored and relies on the selection box for collision instead (except for stairs).

The bug

Referring to the statement above, this means if the player is inside the selection box of the blocks affected (like standing in the corner of iron bars or glass panes), they can throw projectiles through those blocks' bounding boxes, colliding with the opposite side of their selection box. 

Steps to reproduce

1. Place fences in an "x" shape at head height
2. Stand in one of the corners, facing said corner
3. Throw a trident, shoot a bow, or throw an ender pearl

If done correctly, the projectile should collide with the opposite side of the fence's selection box, not the bounding box.

Video Example

https://youtu.be/BilmCeUGV3Y