A configuration for setting a custom error page to Sitecore when a 404, 50x error occurs.

web.config

<!-- Add the following section to your web.config -->
<httpErrors errorMode="Custom" existingResponse="Replace">
  <remove statusCode="404" subStatusCode="-1" />
  <remove statusCode="500" subStatusCode="-1" />
  <remove statusCode="501" subStatusCode="-1" />
  <remove statusCode="502" subStatusCode="-1" />
  <!-- Response the not found page (without changing a URL) -->
  <error statusCode="404" prefixLanguageFilePath="" path="/notfound" responseMode="ExecuteURL"/>
  <!-- Redirect to the static error page -->
  <error statusCode="500" prefixLanguageFilePath="" path="/error.html" responseMode="Redirect"/>
  <error statusCode="501" prefixLanguageFilePath="" path="/error.html" responseMode="Redirect"/>
  <error statusCode="502" prefixLanguageFilePath="" path="/error.html" responseMode="Redirect"/>
</httpErrors>