I vote for Joe Devlin's showing up at the pier and then trying to tell Columbo he's there because he got interested in the code numbers on the paper and started investigating the murder himself! LOL!

"Suction"

The heater was busted on the plane
Which caused the windows to fog up
Which caused Tommy to have to open the window(s)
Which caused suction to suck all the charts and maps out the window(s)
Which supposedly explains why there are no ashes in the pilot's briefcase

. . . especially in a non-pressurized cabin as I'm sure that little crate didn't have.

Another good one is Jose Ferrer's assumption that a criminal would know the chemical formula for heroine.