Square or triangle elements are mostly used to generate a mesh for a simulation.
When two or more nodes of one element fall on the same coordinate the element is called a degenerated element.
This way a square can become a triangle, a line or even a point.
Depending on the used program the multiple nodes will be joined automatically to one node or need to cleaned up manually.
Degenerated elements exist in volume elements as well.
A hexahedron can degenerate to a prism or tetrahedron.
Degenerated elements lead to wrong results in the simulation or even block the start of an simulation.