I know the general phenomenon is that rendering of previews differs from rendering of posts, but, presumably, the goal is to minimise unnecessary discrepancies—and that, in particular, non-esoteric TeX, in an "I know it when I see it" sense, that renders in an external compiler without special packages should render in posts.

The following equation appears in @SidharthGhoshal's answer to Is there a summation method where the divergent series $S = U_0+U_1+U_2+\dots$ converges to a finite value(V2)?:

```
U_n = M\left(c-\frac{1}{n+2}\right)-M\left(c-\frac{1}{n+1}\right)\label{1}\tag{$U$} = \frac{m_0}{\sqrt{1 - 1 + \frac{2}{c(n+2)} - \frac{1}{c^2(n+2)^2}}} - \frac{m_0}{\sqrt{1 - 1 + \frac{2}{c(n+1)} - \frac{1}{c^2(n+1)^2}}}
= m_0 \left( \frac{1}{\sqrt{\frac{2}{c(n+2)} - \frac{1}{c^2(n+2)^2}}} - \frac{1}{\sqrt{\frac{2}{c(n+1)} - \frac{1}{c^2(n+1)^2}}} \right) = m_0 c \left( \frac{n+2}{\sqrt{2c(n+2) -1 }} - \frac{n+1}{\sqrt{2c(n+1)-1}} \right) =
```

I have to admit that I haven't been patient enough to count that braces and `\left(\right)`

pairs match, but it seems to me, if they didn't, then this shouldn't render at all anywhere—and it renders just fine in preview:

and in an external TeX compiler; but it doesn't render in the post:

Sometimes such errors can be fixed on refresh, but that doesn't seem to be happening to me.

`\label{1}\tag{$U$}`

- but the problem that the same label is used twice on one page only manifests when both the question and the answer are displayed. This is what I recently edited. $\endgroup$causedby the label probably has little to do with whether the labelbelongs. $\endgroup$