The user-defined X-Pad response-header field is a work around introduced in 1997 by Apache to fix a bug in Netscape Navigator version 2.x, 3.x and 4.0b2.
This bug made Navigator think an image was invalid if the HTTP response header terminating CRLF occurred at byte 256 or 257. If the server detects that this is the case, it adds the X-Pad: avoid browser bug header to avoid triggering the bug.
Since Netscape Navigator 4 is long gone, this header is not useful anymore. Still, many web servers are sending it.
For more detailed information please check the Apache Changelog