If you take weakness potion, you cant destroy ender crystal (enderdragon regeneration).
Steps to Reproduce:
1. Go to end
2. Take Weakness potion
3. Try to destroy Ender balise.
4. Nothing
Linked issues
Comments
But then you can destroy them with snowballs and eggs which don't do any damage either.
I agree, that seems a bit inconsistent.
I think you can no longer destroy crystals with snowballs or eggs. I've tried it.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

A weakened player doesn't do 0 hearts; it does no damage at all.
A weakness potion reduces your melee damage by 1 hearts. A melee punch is only half a heart, so this seems to me to be working as intended.