Without going into specifics about the final gottcha (Heaven knows cassa doesn't want to be a "SPOILER" ) I think Columbo (or who ever did it) makes the piece of evidence look quite authentic..and at this point Roger is sinking deeper and deeper into his complicated plot...so maybe his mind was a bit rattled and he wasn't thinking clearly or rationally.

The final scene of "Short Fuse" was very well directed, well coordinated with the flashback to the opening darkroom scene and the building tension. Terrific work by Edward Abroms, who also did an excellent job directing the final scene of "The Most Dangerous Match" one season later.