Recursive Mad Covers
"The Droste effect is a Dutch term for a specific kind of recursive picture. An image exhibiting the Droste effect depicts a smaller version of itself in a place where a similar picture would realistically be expected to appear. This smaller version then depicts an even smaller version of itself in the same place, and so on. Only in theory could this go on forever; practically, it continues only as long as the resolution of the picture allows, which is relatively short, since each iteration exponentially reduces the picture's size. It is a visual example of a strange loop, a self-referential system."
Three instances involve the magazine itself repeating into infinity.
On #101 Alfred's Mad magazine hides his Shakespeare book and that image repeats again and again in his hands.
Simon Cowell calls the Mad-reading audience "morons" over and over again on #440.
Mad Special #95 shows Dave Berg's doctor holding the very issue he's holding over and over...
Another example, not quite being the full Droste effect but showing some potential recursion, is with Alfred pulling the rabbit out of the hat on #182. The rabbit then pulls Alfred out of another hat within which Alfred has his hand on yet more rabbit ears. If we assume this happens infinitely then we can make the case for this cover fitting the definition.