What does it mean when no HTTP code is returned

I'm trying to evaluate why some of our attempts to load a page are failing and for the most part, we get standard codes back but in a small per开发者_开发技巧centage of cases we get no code at all. Does anyone have any ideas why this might be the case?

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I know 3 reasons:

1. Many webservers can execute closesocket with out responce.
2. Many developers ignore most of the features and protocol codes for HTTP when develop their web-servers. This is because all modern HTTP is methods POST and GET and Codes 403, 404, 200.
3. Bugs