Almost everyone here knows about Arlene Martel being edited out of "Double Exposure," but I always wonder about a few others who are in the credits.
One is William Christopher in "Mind Over Mayhem." Forgetting his face, it's almost impossible to miss his voice, but I've never recognized it.
Another is Michael Pataki in "Etude in Black." I can never spot him, and he's an easy actor to notice.

There are also actors who are still there but might have had bigger parts that got whittled down.
One is Irene Tedrow, as the socialite in the kind of flashy clothes in "Negative Reaction." She's a well-known character actress, but she's only in it for about ten seconds.
Another is Sorrell Booke in "Swan Song." His character is interesting, but almost all he gets to do is listen to Columbo and Tommy.

Are the first two actors missing entirely?
There's probably no way to find out about the second thing (whether the other two had bigger parts), but I always wonder about it.