- John Wilkinson filed bug #2155496

(http://curl.haxx.se/bug/view.cgi?id=2155496) pointing out an error case
  without a proper human-readable error message. When a read callback returns
  a too large value (like when trying to return a negative number) it would
  trigger and the generic error message then makes the proplem slightly
  different to track down. I've added an error message for this now.

lib/transfer.c

@@ -152,6 +152,7 @@ CURLcode Curl_fillreadbuffer(struct connectdata *conn, int bytes, int *nreadp)
   else if((size_t)nread > buffersize) {
     /* the read function returned a too large value */
     *nreadp = 0;
+    failf(data, "read function returned funny value");
     return CURLE_READ_ERROR;
   }