It's a known glitch. It happens when you delete a map on that slot and create a new map. It will recycle small bits of the previous map creating weird, impossible stuff.
Rollback Post to RevisionRollBack
"If tyranny and oppression come to this land, it will be in the guise of fighting a foreign enemy. The loss of Liberty at home is to be charged to the provisions against danger, real or imagined, from abroad." - James Madison
This happen to anyone else?