I have come across what I believe is the first instance of the Fourth Wall being breached. The Fourth Wall, for the uninitiated, is the unwritten rule that comic book characters do not acknowledge that they themselves are in a comic book. It is an unwritten rule, established between the author and the reader, and it helps to maintain the fiction that the comic book characters are acting out a real-life story.
In the 70s, this rule was broken regularly as comic-book artists experimented and broke new ground, but this is the first instance I have found of this rule being broken in the 60s.
In this cover (from Jan 1968) we see Lois Lane ripping part of the cover of the comic, to establish the fact that she is no longer Superman's girlfriend. This action, of course, is technically impossible, and by tearing a section of comic out, Lois breaches the fourth wall. The cover artist for this comic was Curt Swan, my favourite Superman artist. If anyone knows of any earlier breaches than this I'd be really interested to see them.