This message is generated when mesh elements are specified that self-intersect, have a wrong ordering, or have no area:

To fix this, the correct element ordering needs to be specified:

This message may also be generated during interpolation:

The issue is that because of numerical inexactness, a triangle with 0 area has been generated during mesh creation:

Note that three triangles have been generated and one has poor quality:

Visualize the generated mesh:

Slightly perturbing the input data fixes this:

Inspect the quality: