Someone who calls a tail a leg is either trying to add to the category originally described by "leg" (turning it into the category now identified with "extremity" or something like that), or is appropriating a word ("leg") for a category that already has a word ("tail"). The first exercise can be useful in some contexts, but typically these contexts start with somebody saying "Let's evaluate the content of the word "leg" and maybe revise it for consistency." The second is juvenile code invention.

What about if evolution repurposed some genus's tail to function as a leg? The question wouldn't be so juvenile or academic then. And before you roll your eyes, I can imagine someone saying,

"How many limbs does a mammal have, if you count the nose as a limb? Four. Calling a nose a limb doesn't make it one."

And then realizing they forgot about elephants, whose trunks have muscles that allow it to grip things as if it had a hand.