Question on Export: 2 random clips replace 2 in the timeline during export.

Taking Jim Simons and Ann Bens comments into account, I arrived at a solution to this issue.  In both cases where the clips in the export were from another part of the footage than those I placed in the timeline, I determined that the project clip had been corrupted.  Each had the wrong timecode numbers, which accounted for the wrong clips appearing in the export, though not in the timeline (which remained correct.)  Since the timeline showed what I had intended, it was not until after several hours that I noticed that the timecode numbers were off.  Deleting the rogue clips and re-importing from the original media solved the problem.