In Doctor Who, the Silence (from the Impossible Astronaut 2 parter) were able to edit themselves out of peoplea person's memory. However, surely when a normal human saw one they would scream? And And then wouldn't surrounding humans wonder why this person was screaming, looked at screamed/etctake a look and scream as well, until everyone realised they were there.?
Also, if they've been around for so long, why haven't we seen them before, say in the episode where the Daleks are in the Empire State Building when its being built?