This is an old and everyday enjoying bug :

Make 2 lines with a corner angle slightly higher than 90 (common case on plastic parts with draft to remove from mold).
Make a close parallel line (blue on sketch)
Impossible to use corner tool, unless the distance is sufficient (red line works).
See sketch and file

Thanks Chuck,
Did you try to corner the lines on my example file ?
It is about small small gaps...
Or maybe it's just a Mac thing ??
JL

In the 2D V8 the corner tool worked on the red line to the left. The other gaps were to small. I could extend the other lines and so on. I could see where this problem could fool somebody into thinking they have a closed profile and then would have trouble using the extrude tool because of the opening.
I like using the region trim tool and I make a habit of crossing lines I use for profiles and then region trim them. I also make much use of the snap to "work plane" tool so all lines are on the same plane and will close up and trim.
Chuck