Extra edge that's protruding

Hi I made a square 12 inch by 12 inch then offset it inward at 0.25 inches, then extruded it .25 inches, and then I made another square 12 inches by 12 inches and extruded that surface to .02 inches. When I went to join the surfaces together there was this extra piece hanging off and I don’t know why that was. Can someone explain why that extra material is there please? I will post a photo

Hello- please post a Rhino file with your object.

-Pascal