TIG describes my issue exactly. The first thing I tried was deleting the 'edge', but was frustrated to find that didn't help. I'm not saying that highlighting the edge is wrong - but highlighting the problem face would have been more helpful to a know-nothing-noob like me

(I actually had to install the Manifold extension and run that for it to tell me what the problem was).

Its OK - thanks for looking - but I figured it out! Internal faces! Could SI highlight the face rather than the edge? - that would've given me more of a clue...

Could someone please help a newbie out? Solid Inspector tells me something is wrong with the edges of the 'shelf' in this model, but I don't understand what it could be. It all looks 'ok...?

What is wrong?