> SET_PT_BOTH should only pay attention to `invisible' in the case where
 > there is also `intangible' properties involved.

The emphasis is on _should_ I presume.

 > IIRC you said this
 > thread was not using intangibility but only invisibility, if so, then
 > it's a bug in SET_PT_BOTH.

Please have a look.  I attached a short version of my canonical file.
With Emacs -Q put this in scratch, eval the progn, move to the beginning
of the third visible line where it says "line7", and type C-p twice.
The (forward-line -1) of the second C-p makes the "bug" apparent
(although it occurs already in the first C-p).

 > If you do use intangibility, then indeed all bets are off, and
 > odd/undesirable results are to be expected (yes, I'm biased).

I hope my attachment makes it clear that I don't use intangibility.

martin, who fully shares your biases in this regard.