# MathJax rendering in preview but not in actual post?

So I just passed the 2,000 rep threshold this morning and, eager to try out my newfound powers of automatic editing, I went ahead and looked for the first post I could find that could use some prettying up. I quickly found a post that was well written but had some funky-looking italic math formatting because OP apparently was unsure how to get their $$\operatorname{Hom}$$'s to look nice. I went ahead and fixed it up, adding a \ in front of every Hom I could find, and verified that this was valid syntax by noting that it rendered nicely in the preview. Alas, it was not valid syntax, as when I later looked at the edited post I saw a bunch of red 's everywhere. Well, that was embarrassing.

I immediately began fixing up my mess by replacing all by \Hom's with \operatorname{Hom}'s, achieving the desired result. But seriously, why would my LaTeX look nice and rendered in the preview and then atrocious in the actual post? I feel a little betrayed here. Can anyone explain what is going on?

• Just to make this more specific - in case somebody wants to do some detective work about this - are we talking about this question: How to show that $\text{Hom}_R(A\times B ,M)\cong \text{Hom}_R(A,M)\times \text{Hom}_R(B,M)$ when $A, B$, and $M$ are $R$-modules? – Martin Sleziak Nov 21 '18 at 20:31
• This post might be (to some extent) related: Mathjax bug while editing an answer.. I did not find \Hom defined in other posts (an older revisions of them) on that question. (Deletion of such post would be an explanation.) Maybe it is possible that some post that defined \Hom in the title was shown among related questions? (It is very bad practice to define such stuff in the title - but apparently there is at least one post that does define \Hom in the title: math.stackexchange.com/q/2986920 ) – Martin Sleziak Nov 21 '18 at 20:36
• @MartinSleziak the idea with the title in the related is intriguing. I'd not have thought about looking there. Would it spill over? – quid Nov 21 '18 at 20:40
• @quid I would guess that linked posts behave similarly as related posts - so probably it is not difficult to add a temporary comment to add the link to the above post somewhere and test this. (If somebody has the time and energy to do so.) However, it's nearly impossible to find out whether some particular post was shown among related questions - since they are generated by the software and they sometimes change. – Martin Sleziak Nov 21 '18 at 20:42
• @MartinSleziak I'll do it. – quid Nov 21 '18 at 20:45
• @MartinSleziak Brilliant theory. It does behave exactly like described. When that question is linked it works in preview, but not once posted. // Sorry for the rollbacks on main. – quid Nov 21 '18 at 20:51
• @MartinSleziak Yes, that is the question – Monstrous Moonshiner Nov 21 '18 at 20:53

$$\DeclareMathOperator{\Hom}{Hom}$$