If you run workspace.Terrain:GetTouchingParts() you will get an error:
GetTouchingParts is not a valid member of Terrain
However, this is not the case. You can do workspace.Terrain.GetTouchingParts(workspace.SomeOtherPart). And also if you do workspace.SomeOtherPart.GetTouchingParts(workspace.Terrain), the error message will say the same thing, even though you did not try to index Terrain with GetTouchingParts at all.
Expected behavior
A better error message, such as:
Terrain does not support GetTouchingParts()
Instead of incorrectly stating that you tried to index a member that does not exist.