AuFCL writes, at https://www.mediawiki.org/wiki/Topic:S2x174x44fnr5xx7 :

when previewing results of modifying a <math> expression on English WikiSource with HHVM enabled, this error is presented:
Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.):
\scriptstyle{i=\frac{702\cdot25}{2\times10}=35\cdot11,\quad\mu=1\cdot738} .

Notes:

• The second line of the error message is simply informational regarding the input LaTeX string and is not likely pertinent to this report.
• The error reliably goes away when HHVM is deselected from Beta choices.
• Reliably returns when both HHVM reenabled and the previewed formula is tweaked to force re-evaluation.
• There is no problem displaying cached results irrespective of HHVM state.
• There may be further significance in the fact that <code< does not appear to have been parsed by the error processor.
• Verified similar results on English WikiPedia.

(In reply to Michael M. from comment #0)

I don't know whether just installing texvc on the respective servers is
enough, or if there is a real issue with the extension Math under HHVM. If
there is, please move this report accordingly.

I'm pretty sure it just hasn't been built on the hhvm servers that were reinstalled

scap-recompile

Change 162795 had a related patch set uploaded by Ori.livneh:
Use Debian-packaged texvc on Trusty app servers

https://gerrit.wikimedia.org/r/162795

Change 162795 merged by jenkins-bot:
Use Debian-packaged texvc on Trusty app servers

https://gerrit.wikimedia.org/r/162795