The Bug
Mobs can be flung out of the world border in the nether if hit with a weapon that has knockback (any level, as long as it can fling the mob outside the border)
How to reproduce
Go to the world border (or set the world border to a place very close to where you are)
Give yourself a weapon with Knockback.
Find or spawn a mob right at the edge of the world border.
Hit the mob with your weapon. The mob should be flung outside of the world border.
Attachments
Comments 3
This appears to be a duplicate of MC-121997, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.
Thank you for your report!
We're tracking this issue in MC-121997, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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Looks like they can also walk out of the border too.