An invalid connection occurs when an edge of a triangle is shared by more than two triangles.

Micromine's advanced topological checking will allow an even number of triangles to share the same edge. This is valid when two cubes touch on an edge.

An odd number of connections is generally considered to be invalid. When a third triangle lies inside a solid, it is referred to as a hanging sheet.

You can use Wireframe | Utilities | Advanced Repair to remove them.