74. The HTTP spec allows headers to be merged and become comma-separated

  instead of being repeated several times. This also include Authenticate: and
  Proxy-Authenticate: headers and while this hardly every happens in real life
  it will confuse libcurl which does not properly support it for all headers -
  like those Authenticate headers.

docs/KNOWN_BUGS

@@ -3,6 +3,12 @@ join in and help us correct one or more of these! Also be sure to check the
 changelog of the current development status, as one or more of these problems
 may have been fixed since this was written!
 
+74. The HTTP spec allows headers to be merged and become comma-separated
+  instead of being repeated several times. This also include Authenticate: and
+  Proxy-Authenticate: headers and while this hardly every happens in real life
+  it will confuse libcurl which does not properly support it for all headers -
+  like those Authenticate headers.
+
 73. if a connection is made to a FTP server but the server then just never
   sends the 220 response or otherwise is dead slow, libcurl will not
   acknowledge the connection timeout during that phase but only the "real"