I find myself repeatedly today not finding pages with mwgrep and/or with on-wiki insource: searches for things that I do find with the regular search index.
For example, the following all were not found by mwgrep and not found by on-wiki insource: searches for the cited pieces of text
- pl.wikipedia.org MediaWiki:Gadget-nuxedtoolkit.js: UsabilityInitiative or combined.min.css was not found. – fixed by edit
- commons.wikimedia.org MediaWiki:Edittools.js/dev textSelection was not found. – fixed by edit
- pl.wikipedia.org MediaWiki:Gadget-RTRC.js text RTRC not found.
https://pl.wikipedia.org/w/index.php?search=insource%3ARTRC&profile=advanced&ns8=1 matches MediaWiki:Gadgets-definition and MediaWiki:Gadget-RTRC, but not MediaWiki:Gadget-RTRC.js.
The same search without insource: does return all three.