It was discovered that a map (brest/brittany/enigme.5, upper right with wraiths and beholders) had two monsters stacked on top of each other or both taking up space when they should not be allowed to stack. Gridarta did not alert on this issue.
Feature request to detect stacked monster instance(s) and show it as a warning in the map editor.
Enclosed is the live map with this issue.
The check has been implemented. It is included in the Crossfire editor variant.