To quote the following 2013 message from this mailing list
http://lists.gnu.org/archive/html/emacs-orgmode/2013-06/msg00841.html

> This is quite strange because I can debug skipping functions
> for tags-todo blocks, but for some reason I cannot debug skipping
> functions for agenda blocks.

I try to debug a skipping function for agenda. debug-on-entry has no
effect. debug-on-entry org-agenda-skip-eval had an effect once but I
can't reproduce it; besides, another message
http://lists.gnu.org/archive/html/emacs-orgmode/2013-06/msg00854.html
from the same thread reads:

> It would be great except for the following: as soon as I use the
> debugger, my function works as intended.

so I won't feel confident I won't be hit by the same bug.

How do you debug agenda skip functions?