If you die with a custom max heath (for example 100) set by an NBT editor, your max health will be back to 20.
How to reproduce?
1) Create a new world.
2) Modify your max health with an NBT editor (for example 100).
3) Load your world and die. You'll see that when you respawn, your max health is back to 20.
Linked issues
is duplicated by 1
Comments 10
Not a dupe, MC-18230 had 2 reports in one ticket which is not allowed. Only 1 of the 2 was fixed, but not this one.
No response in 4 months, assuming no longer an issue.
Dude Talven no... just no... did you just left a bug not to be fixed? Couldn't you reproduce the bug? I'll reproduce the bug soon to proof this bug's still an issue. If no response in 4 months, it might be because it was LEFT OUT.

We ask if it is still a concern. If we don't get a reply in a few months, we close it. If you reply after we close it, we can reopen it.
Then good, but I still say you could reproduce it + I confirmed the bug in 1.7.2 and snapshot 13w48b

And: Is this reproduceable in Vanilla MInecraft without any 3rd party programs?
There isn't a command like that on MC at the moment, but certain map makers (don't know which ones atm) modify the player's custom max health. The extra hearts shown when you modify your max health or you get a health boost or something, were shown for something I believe.

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. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been deliviered.
Dupe of MC-16773.
Duplicate of MC-18230, and I didn't know because of its title, I tried looking for it, but nothing though.