Jean-Paul M. writes: I have a Canadian penny with only one side printed, it is the face of the Queen but I can’t say which year it was emitted.
One sided coins indicate that something material has come between the coin blank and the die. It is also possible for dirt or gas to be trapped in the coinage strip when it is rolled out. When the coin is struck the defective side comes apart leaving essentially a half of coin with only one side with design. Laminations are more usual on clad strips but I think it is possible even on pure copper flans.