Steps to reproduce:
- Make sure you have HHVM enabled.
- Visit any page with incorrectly nested HTML (e.g. [[de:Wikipedia:WikiCon 2015/Umfrage]], if that page gets fixed, just use the sandbox and open some unclosed <div>s)
- Purge the page.
Tidy fixes the incorrect nesting/missing closing tags etc.
The page layout is messed up, caused by the missing closing tags.
Note that once the incorrect HTML is saved to cache, it also affects non-HHVM views.