The mender targeted a wolver, which had somehow teleported itself off the map boundary for a moment; the heal zone went underneath the wolver.
Two issues might be clear from this: A mender's heal zone will spawn on the same level as it's target, even if that area cannot be reached by players. If the heal zone is under the floor, the players will not be able to tell exactly where it is, as it will be at least partially hidden and not directly under the area it is healing.
Also, wolvers are capable of teleporting slightly off-map. This may lead to issues like unreachable drops appearing.
