Bad request on clicking a post.

Sometimes when I add a post, it shows on homepage all right, but when you click it on your homepage, it gives you an error.

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Your browser sent a request that this server could not understand.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.
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It is weird because I don\’t get it every time I post. It occurs and it doesn’t on another post. I even tried to delete a post with this error message and add a new one with basically same message with very little change. Well that didn’t work either.

Does anyone have similar problems? Help out a wordpress newbie please.