12D

Explanation
A program requested an overlay.  When trying to
answer the request, the overlay supervisor found that words 3 and
4 of the overlay segment table are incorrect.
The overlay segment
table is used by the overlay supervisor; it resides in the load module
and is in the same storage protection key as the application program.
Note: Overlay
programs run only in 24-bit addressing mode, not in 31-bit addressing
mode.

System action
The system abnormally ends the program that requested
the overlay.  The system writes an ABEND dump, if a dump DD statement
is provided.

Programmer response
Check the program for errors that caused
the overlay segment table to be incorrectly modified.  Correct the
errors; then assemble, link edit, and run the program again.

Source
Overlay supervisor