As you can see, the health bar is visible, but the hero (in this case Callidus) has glitched behind the container and cannot be hit by basic attacks(as evidenced by the sparks and bullet marks on the container). Skills will however hit the hero regardless, so it does not make her unkillable.
I think this occurs when a hero is staggered and is pushed backwards through the container. The stagger glitch can occur in any map, but the hero only gets stuck behind a wall in the dock map.
Developers are aware. Apparently it happens when the cover is broken and the hero gets staggered at the same time. Bonus points if the hero was stunned during stagger animation.
Heroes can also get stuck on Atrium (only midlane heroes)
You should report it ingame, though. Support can access exact information of the match where you experienced this, and may help them pinpoint the problem
Thanks for the heads up. We try to catch/prevent these kinds of issues.
It seems to happen the most with the containers, I had a few stuck in them, same happened in the bounty where galante was stuck