Err: The specified CGI application misbehaved by not returning a complete set of HTTP headers ...

While: Using fpdf in PHP

Error:

    CGI Error The specified CGI application misbehaved by not returning a complete set of HTTP headers. The headers it did return are:
(or)
    PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 6144 bytes) in C:\Inetpub\wwwroot\ult\fpdf\fpdf.php on line 51

Reason: 

    fpdf.php file or related included files may contain error or wrong codes.

Solution:
    Replace fpdf folder by fresh one to makesure the error cause and review the last modified file inside the fpdf library.