This seems to be a not rare occurrence around the phase boundaries of functions. I had a similar problem with the atan2 function in the PL/I - Fortran library

This seems to be a not rare occurrence around the phase boundaries of functions. I had a similar problem with the atan2 function in the PL/I - Fortran library on the CDC 6600. One of my testers was stepping across the boundary between valid and invalid arguments and there was a single binary value that blew up. I was the compiler lead so it got fixed. Any of you remember the 486 floating point problem?