Tell ASP.NET never to use %UnicodeValue notation when URL encoding by putting the following appSetting in web.config:
<add key="aspnet:DontUsePercentUUrlEncoding" value="true" />
Sometimes, such as when calling NameValueCollection.ToString(), that value gets url encoded for you. However, the url encoding in .NET defaults to a %Unicode notation, which, according to MSDN's own warning now attached to the obsolete-as-of-4.5 HttpUtility.UrlEncodeUnicode() method, "produces non-standards-compliant output and has interoperability issues." Therefore, even if you are targeting .NET Framework 4.5 in your project, NameValueCollection.ToString() will still use that obsolete method, and you will get %u00XX style encoding in your URLs.
Telltale signs of "interoperability issues" include the word Français being encoded Fran%u00e7ais, which then blows up the search engine you lovingly built that runs on Java and Apache Solr.