Error 400 when downloading user image


I’m trying to download a user image profile, i’m using the twitter api search to get the normal picture: e.g. id)/(another id)_normal.jpeg.
but for some urls, without anything in common as much as i can see, i’m getting an error while trying to get the image.
sometimes it works, sometimes it doesnt, and i can’t point at something specific that causing the problem at the client side.
i’m using c# to download the image, and the error is:
System.Net.WebException: The remote server returned an error: (400) Bad Request.
at System.Net.HttpWebRequest.EndGetResponse (IAsyncResult asyncResult)

as anyone else having same sort of troubles accessing user profile images?