The CDC 6500 at Purdue occupied the basement of the then newish Math Science building. Batch job output appeared in bins along the hallway where one could look through glass at the computer itself. Across the hall was the consulting office where staff would help debug programs, usually by explaining error messages.
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Distraught Fortran programmers would wait in line for me to explain their error messages. For example:
OPE ILLEGAL FILE NAME
OPE was a Peripheral Processor program for opening disk files. Systems programmers would call PP programs by writing "OPE"+address into Central Processor memory location zero.
If a Fortran programmer called a subroutine in the wrong number of arguments, the compiler would substitute zero for missing arguments. Fortran being call-by-address would leave an accidental entry into the PP space.
The subroutine had only to write a normalized floating-point number in the range 1/2-1 into that missing parameter to get this exact error. This because the exponent of this range of floats spelt OPE. wikipedia 
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