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There are two code paths that can lead to ResourceLoader.php calling tryRespondLastModified:

ResourceLoader::respond
  if ( $wgUseFileCache )
    ResourceLoader::tryRespondFromFileCache
      ResourceLoader::sendResponseHeaders
      ResourceLoader::tryRespondLastModified

or

ResourceLoader::respond
  tryRespondLastModified

Only one of these paths calls ResourceLoader::sendResponseHeaders.

The other path will set a HTTP 304 status and Status: 304 header, but no other headers.  The lack of the Expires header in particular causes a variety of odd problems when using (at least) mod_proxy/mod_disk_cache as an apache-based caching reverse proxy.

Using mod_proxy and mod_disk_cache from apache2 pre-2.2.18, this leads to the proxy returning erroneous 304 responses to clients who sent unconditional requests (see https://issues.apache.org/bugzilla/show_bug.cgi?id=45341).

After the fix/workaround added in 2.2.18, mod_proxy will now correctly send the cached content back to the client, but will then immediately purge the page from the cache, which is not ideal.

I fixed this locally by moving the call to sendResponseHeaders from the first call path after the call to tryRespondLastModified, then adding a call to sendResponseHeaders in tryRespondLastModified itself.