Used to run into this error frequently with a piece of software I supported. I don't remember the specifics, but it was your basic C program to process a record

Used to run into this error frequently with a piece of software I supported. I don't remember the specifics, but it was your basic C program to process a record-delimited datafile. A for-loop with an fopen that didn't have a corresponding fclose at the end of the loop. For a sufficiently large datafile, eventually we'd run out of file handles.