Hi Chris, I've attached a fixed up and filleted version.

So the actual bug was in some previous step that kind of collapsed that small surface fragment's edges down into nothing.

I'll see if I can track down where that was happening, it can sometimes be difficult to find exactly what caused something like that looking at just the final result though.

You may want to run the Rebuild command on curves before constructing a bunch of stuff out of them, especially if they're going to be filleted - the Rebuild command can simplify the curve structure and combine pieces together so there aren't so many little separate segments. When you have little surfaces that are small enough to where they are approaching the fitting tolerance it can cause some difficulties in handling them, it can cause some kinds of algorithms to think that opposite sides of the surface are close enough to get fused together and stuff like that.

- Michael