I have noticed a curious bug in the way MathJax is setup on MSE. MathJax has feature to zoom its formulas via "Math Settings -> Zoom Trigger" menu option. I normally use this menu option and set double-click as the Zoom trigger and zoom setting to 200% so that double clicking zooms the content to twice the size (note that double-click opens up a new window where it shows the zoomed content) . It is helpful in viewing formulas having lot of small subscripts or superscripts.
However this zoom splits a single line formula into multiple lines and this makes it difficult to grasp the formula when viewed in zoom mode. I use MathJax in my blog (in the blog the zoom trigger is defaulted to double click) and there I don't see this problem. Probably some settings in MathJax is not proper and should be fixed by modifying the css files.
Image of the formula given by Asaf Karagila in comments is shown below:
Another image showing the same formula in one line (I don't know what triggers the split shown above and BTW split is seen most frequently and one line is rare):
Update: I use the following configuration for mathjax in my blog css and I don't find this issue on my blog. Perhaps that may help:
<script type='text/x-mathjax-config'>
MathJax.Hub.Config({ messageStyle: "none", TeX: { equationNumbers: { autoNumber: "AMS" }, extensions: ["autobold.js"] }, tex2jax: {inlineMath: [['$','$'], ['\\(','\\)']]}, processEscapes: true, menuSettings: { zoom: "Double-Click" }});
</script>
<script src='http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML'></script>
Further Update: I checked that this issue is specific to MSE only. The MO site does not have this problem. You can see the equation $(1)$ in question on MSE and the same question on MO. This I find very difficult to accept.
\sum x_n=\sum\sum y_{nk} = \iint f(x,y)\operatorname{d}xy=\frac{\aleph_{\omega+1}}{2\pi}=\aleph_{\omega+1}=\aleph_\omega^{\aleph_0}=\infty
$\endgroup$