Obvious, isn't it? Yet, not totally. First off, within the red square you see the vent offset from the tank. What is not obvious is that there is something missing: a draft diverter. The vent doesn't fasten directly to that hole. The draft diverter goes in between.

So what happened?

What happened was people -- tenants, in this case. They piled a bunch of stuff into the closet, on top of and around the water heater. They knocked the vent off the draft diverter and the latter onto the floor.

Ironically, they were potentially committing suicide. The closet was inside their apartment. The exhaust gases were going the same place, for the most part. People do die from carbon monoxide poisoning, as well as suffering other ills.

What's the solution? Locks. Yes, if you lock the closets, children can't open drain valves or shut off controls or burn themselves. And thoughtless adults can't create hazards.

Locks! Great idea!