ALTER

I discovered the ALTER command in COBOL, in this note on eliminating that command, allowing self-modifying code. A couple of quotes:

ALTER changes the destination of a GO TO statement elsewhere in the program. It complicates the task of eliminating GO TO, especially if you don't notice that the ALTER is present.

and even worse:

In the presence of segmentation, ALTER can behave in unexpected ways. If an independent segment contains an altered GO TO, and the segment is overlaid and then reloaded, the GO TO is reset to its original destination.
If you ever encounter this form of pathology, proceed with extreme caution. You have found either a bizarre and subtle bug or a vicious marauding style of programming. Quite possibly both.