Practice cube 7: Are these pieces really incorrect?

I’m sure the practice cubes, being the practice cubes, have been vetted thoroughly.  However, I would like assurance that the pieces marked as incorrect really are.  More than that, I would like to know how I’m supposed to tell these pieces don’t connect.  They very much look like they do to me.

(I can see the mistake I made with the piece marked missing.  I should have been paying attention to the change when I switched slices.)

Edit: I hope my screenshots give enough information to answer my question.

Hey onexused you’re right that that little red piece does actually belongs, it seems you found something we missed.  Thanks!