If you kill a wolf and give it a bone during its death animation, you get a tamed dog that is red and sits diagonally, jittering back and forth in its death animation. It behaves like a normal dog, you can make it stand up and follow you, etc., it just looks like it's dying all the time.
After reloading the world it's just a regular dog.
Tip to reproduce this: Put a lot of wolves in a 1x1 hole, take a weapon in your main hand and bones in your offhand and spam-click both buttons. For easier results enchant the weapon heavily or give yourself a big strength effect. You can also put
/summon Wolf ~ ~1 ~ {Health:0}
on a clock and give the dying dogs bones.
Idea for a fix: Someone at Mojang could do complicated stuff with the wolf->dog conversion, but there could still be other bugs with perma-dying mobs in the future. Better just let dying mobs not have a hitbox. Maybe just completely replace the death animation with a particle animation.
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Oh, that's such a stupid bug (in JIRA, not in Minecraft). When I edit the text field, there's an end of the line after the last tilde. For some reason it inserts a new line then in the text instead of just letting the browser do it properly. So when you mark and copy it, it gets copied without a space between ~ and { . You have to put a space there to make it work.

Ok, no, it's apparently something with the { sign. Don't know what happens there.
the command! to you speak english, [LOL]!?

As I said: Put a space between the last tilde (~) and the opening curly bracket ({). The rest of the text we an explanation, why must copying and pasting doesn't work.
/summon Wolf ~ ~1 ~
{ Health:0}
like this?

The {
and }
need to be escaped in Textile by putting backslashes before. See the formatting help here
To format commands, simply enclose the command in {code}
tags:
/summon Wolf ~ ~1 ~ {Health:0}

Related to MC-2376.

Relates to MC-76416 (same reason)
/summon Wolf ~ ~1 ~
{Health:0}
output says {Heath:0} isn't a valid number