Odd folding in mesh

I just recently came back to Blender after a long hiatus. I felt like modeling a Beretta for fun, but I ran into this odd folding in the mesh when I try to make a face. It initially happened to all four edges, but after fooling around with it, I managed to fix it on three of the edges. Right now, it’s a bit frustrating as I’ve searched it up, and have yet to find a solution, so I decided to ask here. I attached both a picture, and the .blend file. Encased within the file is the error color coded as to make anyone attempt at help easier.

It’s a bit hard to see, so I encased the .blend as well. The mirror, and subsurf modifiers are on currently.

Problem.blend (497 KB)

Please & Thank-you. I’ve never run into this problem prior in my years with Blender, so I am quite confused.

Looks like you might have interior faces.

Ah ha! That was it, wonderful. Must’ve forgotten to delete it! Thanks for the help.