Like DaleB says; in the example below, if something other than an 8,9,A,D, or E 
is found in position 1 of a screen the result would be the error you described 
rasika. (this example is from an RPGII pgm converted with help from cvtrpgsrc) 

                                                          
IFilename++SqNORiPos1+NCCPos2+NCCPos3+NCC.................
I          XX        1 C8                                 
I                                  1    1  TYPE           
I                                  2   13  TAXCUS         
I          XX        1 C9                                 
I                                  1    1  TYPE           
I                                  2    3 0LNUMB          
I                                  2    3  LNUM           
I          XX        1 CA                                 
I                                  1    1  TYPE           
I                                  2    8 0OEC            
I                                  2    8  OEE            
I                                  9   15  PNAME          
I          XX        1 CD                                 
I                                  1    1  TYPE           
I                                  2    8 0OEC            
I                                  2    8  OEE            
I                                  9   15  PNAME          
I          NS        1 CE                                 
I                                  1    1  TYPE           
I                                  2    6  SEC            

This isn't really because it's a display file. It's because you have I-specs
with indicator in 21-22 and character tests in 23-46. The record coming in from
PAY004N does not match any of the character tests, so RPG doesn't know which
record it is. You need to check your input buffer to make sure the character
tests match the actual records, or create an extra record identifying indicator
after all the others, to act as a catch all. (But, really, if you're using
display files, and this isn't RPG/II in S/36EE, you should be using externally
described.)

Names record format & file didn't match...

I ve define a program describe display file and compile program properly.But i 
am getting runtime error as below.
                                                                               
 
 Message . . . . :   Undefined record type is found in PAY004N 00000000       
   PAY004F2 (C G S DF).                                                        
                                                                               
  Cause . . . . . :   The RPG program PAY004N in library HPOPY6IS at statement 
  00000000 detected an undefined record type in file PAY004F2. A statement     
  number of 00000000 indicates that an error occurred while reading a primary  
  or secondary file. If file PAY004F2 is a program-described file, a record is 
  read with record-identification codes that do not match any of the record    
  types defined on the input specifications for the file. If file PAY004F2 is  
  an externally-described file, a record is read with a format unknown to the  
  RPG program.