Short answer 1: When you know an artist recorded a cover in a certain way purposely. Maybe to be funny, maybe just to work with the original artist who's in a different genre (damn near anything can be made into a C&W tune), or maybe for the sheer hell of it. You just added an album full of songs like that.

2: When someone who's not a musician covers a track & a train wreck ensues - whether the non-musician sees it coming or not. William Shatner's the classic example. Which leads to...

3. When an artist wants to "try something new", does covers outside their musical bailiwick and hilarity ensues. Usually, the artist's in on it all...usually. Before the internet, this would occasionally be done purposely to fulfill a contractual obligation - "X number of albums" and all that. Cyndi Lauper made this Xmas album once....

4. Covers in other languages or foreigners singing covers in English. For this one, the caveat is that these have to be amusing.

And if I woke up more than 45 minutes ago, I'd have come up with more. And they'd have been more coherent. Meanwhile, back to making Halloween shows.

I think it has to be an exaggerated or extreme different style. There's been rock songs cover by country artists taking rock to country a billion times ie Footloose from Kenny logging to Blake Shelton, BW Stevenson's My Maria covered by Brooks and Dunn and this list goes on forever.. To many this isn't a big enough "jump" for a cover to be added, I think the cover has to be completely jump a genre to be added like Ozzy Osbourne covering BeeGees "Stayin Alive" to be considered.

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e8:


Possibly when it is done in a different style.
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Halfshell:

When is a cover song considered appropriate to be added to demented archives?