It is possible that an error in the program has caused part of the instructions that can be run to be overwritten by data. Other possible causes of an operation exception might be an attempt to invoke an external procedure or other routine that was not incorporated into the running program by the linkage editor, or running a branch instruction that is incorrect because a control block had previously been overwritten. Consequently, it is advisable to check the linkage editor diagnostics to ensure that all requested external procedures and subroutines have in fact been incorporated into the running program, and that any overlay phases do not overwrite any phases that are still active.