One of the most commonly-seen preprints is a large (16x20") 'signed' image of the Apollo 11 crew, shown above. This has also been reproduced in 8x10" form.
Less common, but still seen from time-to-time is this 8x10 litho with pre-printed signatures.
This facsimile cover was created by Fleetwood as a copy of a real crew-signed Dow-Unicover. This is stated on the reverse but if seen in an action listing with only the cover shown it can be very deceptive.
Another Apollo 11 facsimile signed cover to watch out for. Not as common as the above but just as deceptive.
An Apollo 11 facsimile signed card which is often mistaken for the real thing.