I recently found such a surface in a customer’s file:
CP_2024-10-15_Srf.3dm (831.3 KB)
A closed surface?
Which shows the normals in both directions?
And a seam in the middle?
In short, I found out what happened.
It is a sphere, scaled by factor 0 in one direction using the Gumball.
The expectation was a flat surface 
Strange idea.
In the file, I discovered similar instances.
What appeared to be a straight line was actually a closed curve, possibly an ellipse or a circle.
Should it be even possible to create such geometry?
At least a warning could pop up.
Because such elements are valid, so _SelBadObjects/_CheckNewObjects doesn’t fire.
