If you pass a bad url into drupal_http_request (for example http:/example.com (one slash); it fails silently, with only a notice in watchdog with a missing index.
Notice: Undefined index: host in drupal_http_request() (line 808 of /includes/common.inc).
Perhaps there should be a more verbose entry in watchdog to help debug this kind of error.
This is halfway between a bug report and a feature request. Feel free to reclassify.
Curl would squawk noisily with this.