In ibrowse_http_client:make_request/7 there's a call to ibrowse_lib:get_value(content_length, Headers, false) and ibrowse_lib:get_value(transfer_encoding, Headers, false). get_value does not attempt to check if the header exists as a string which ends up causing things like having multiple Content-Length headers set.

http://github.com/cmullaparthi/ibrowse/blob/master/src/ibrowse_http_client.erl#L806