How To Fix AutoCAD Fillet Trims Segment In The Wrong Direction

I've been using AutoCAD since 1992, over 20 years.  This morning I was stumped for a while because while trying to use the Fillet command to create a simple rectangle with lines, the Fillet command was trimming instead of keeping the side of the line I was selecting.
I tried the Fillet command in a new drawing to determine that AutoCAD was indeed working right and it was my drawing that had trouble.  I tried Audit on the drawing.  I tried Wblock * ("ultrapurge") on the drawing.  Neither worked.  The trouble was that my view, my ucs, and the extrusion directionn of my objects were not in parallel planes.  I fixed the problem using the PLAN command, like this: UCS World then PLAN Current UCS, or PLAN Enter.  The plan command is a view command.  It sets your view to match a ucs.  That fixed Fillet!