[Consistently with $\mathsf{ZFC}$](https://math.stackexchange.com/a/853079/28111) there is a forcing which preserves cardinals but whose square does not *always* preserve cardinals - that is, some $\mathbb{P}$ such that for every $\mathbb{P}$-generic $G$ we have $\mathrm{Card}^{V}=\mathrm{Card}^{V[G]}$ but for some $\mathbb{P}^2$-generic $H=\langle H\_0,H\_1\rangle$ we have $\mathrm{Card}^{V}\not=\mathrm{Card}^{V[H]}$.


However, the only way I know how to get this is via a bit of a cheat: find two *different* forcings which are individually "good" but have "bad" product, and then look at their lottery sum. This construction has the drawback that the square of the resulting forcing doesn't always collapse cardinals - we're only guaranteed "bad" behavior in the extension if the two coordinates of our generic lie on different "sides" of the original lottery sum.


I recall$^\*$ seeing a stronger example of this phenomenon, but I can't track it down or reconstruct it on my own:



> 
> Is it consistent with $\mathsf{ZFC}$ that there is a forcing $\mathbb{P}$ such that $\mathbb{P}$ preserves cardinals but $\Vdash\_{\mathbb{P}^2}\mathrm{Card}^V\not=\mathrm{Card}^{V[\langle H\_0,H\_1\rangle]}$?
> 
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$^\*$Actually my *original* memory was that even the weaker phenomenon can't happen, but after it was pointed out to me that it can, I now remember differently. I'm sure eventually my memory will have been right. :P



Good forcings with bad squares
A self-specializing Souslin tree gives you a ccc notion of forcing whose square collapses $\omega\_1$ (See, e.g., the answer to
[Ultrafilters preserved by $\mathbb{P}$ but not by products?](https://mathoverflow.net/questions/251930/ultrafilters-preserved-by-mathbbp-but-not-by-products)). Such trees exist under $\diamondsuit$, and so are consistent with ZFC.


