In 1974 The Beano made a small mistake of repeating the issue number on the front cover. Both issues dated May 4th and May 11th 1974 has issue numbers #1659. It's very unusual for such a mistake to appear as the editor (who at the time was Harold Cramond) would usually spot any errors before they went to press.
This rare mistake happened again in 2011 with the issue that celebrated Dennis The Menace's 60th birthday. Both issues dated March 19th and March 26th 2011 had the issue number #3577. The reason for this mistake is that these were the issues where Alan Digby retired and Michael Stirling took over, although the mistake should have been spotted.
Now, if you're wondering if that means that the current issue number is wrong, well it isn't. After #3577 appeared twice, Beano dated 5th April 2011 is numbered #3579 - meaning that there is no Beano #3578!
It was the same story in 1974. The issue after the second #1659 was issue #1661, so that means there is no Beano #1660.
With thanks to Keith Shenkin for the photograph at the top.
Thanks to Dan Webb for the scan of Beano #1661. Please visit his website about the history of advertising in the Beano: