Whilst checking something buried in a screen procedure within Priority, I came across the following nugget:
SELECT 420, 539 INTO :WRNMSG, :GOTO FROM DUMMY WHERE :RETVAL <= 0;
I wrote a quick procedure which confirmed what I thought this snippet does: if the variable RETVAL is less or equal to zero, then warning message 420 will be displayed and execution will skip to label 539. As it happens, earlier this morning I had written something much less concise than this to achieve the same result, so the find was quite serendipitous.
Here are two more undocumented variables which can be added to one's private toolbox.