Trying to shoot entities with a bow and arrow from close range doesn't work. This also affects other projectiles. Withers shoot through blocks from close range as well.
Occurs when the shooter hitbox overlapping with the mob hitbox, or the mob/player is next to the dispenser
applies to skeletons, players, pillagers, illusioners, strays, wither skeletons with a bow and snow golems for sure; presumably also blazes, ghasts and ender dragons (with a dragon fireball)
It happens when the projectile's "hit area" (area where the projectile hits the entity, not equal to its hitbox) intersects with the entity's hitbox, so it happens not only when you touch the shooting spot, but a bit away from it too
(see the spawn points of the arrows from dispenser in screenshots
and
[media])
Simple reproduction steps:
Place an upwards facing dispenser
Put in an arrow, snowball, fire charge, ender pearl, splash potion, lingering potion, or any item that creates a projectile when fired
Place a pressure plate on the dispenser
Let any mob or player step on it
Observe projectile going through the mob or player that stepped on the pressure plate
Related issues
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This issue duplicates MC-122335.

Was able to re-create this on Minecraft 1.12.2.

Confirmed for 1.13.1.

affects 20w17a

affects 20w18a

affects 20w19a

Can i request the ownership of this ticket?

@unknown sorry for delay - the ticket is yours now

Confirmed for 1.16.2.
Simple reproduction steps:
Place an upwards facing dispenser
Put in an arrow, snowball, fire charge, ender peral, splash potion, lingering potion, or any item that creates a projectile when fired
Place a pressure plate on the dispenser
Let any mob or player step on it
Observe projectile going through the mob or player that stepped on the pressure plate
This is not only for dispensers, but applies to skeletons, players, pillagers, illusioners, strays, wither skeletons with a bow and snow golems for sure; presumably also blazes, ghasts and ender dragons (with a dragon fireball)
You cannot shoot through multiple entities in a row, the first entity whos hitbox that didn't intersect with the arrow's, gets hit.
Compared to the projectile's spawning position (see screenshot named spawn 1.png and spawn 2.png), the entity has 5 pixels on a block of "free space" before it gets hit, if the hitbox is in any of the "free space", the projectile will go through the entity. (see other screenshots)
Can confirm in 21w03a.
Can confirm in 21w06a.

Can confirm in 1.17.1 Release Candidate 1.

Can confirm in 1.19.4

Can confirm in 24w06a.

Can confirm in 24w20a.

Can confirm in 24w33a

This issue additionally causing some projectiles passing through entities when launched directly beneath them and the entity is moving towards the projectile source, as the entity bounding box is already inside the projectile's bounding box.
[media]
Can confirm in 1.21.4 and 25w02a