When I search for a string that is a namespace (including the colon), like Wikipedia:, an error is thrown:
https://en.wikipedia.org/w/index.php?search=Wikipedia%3A&title=Special%3ASearch
"An error has occurred while searching: The search backend returned an error:"
Version: unspecified
Severity: normal
URL: https://en.wikipedia.org/w/index.php?search=wikipedia%3A&title=Special%3ASearch