Sometimes when I try to create new edge between two subd surface points (vertices) it doesn’t create new edge there but creates a new point on top of the existing one and creates two edges instead of one, only that one of these edges is infinitesimal.
And sometimes it simply refuses to create an edge.
What’s up with that?
I cannot grasp the logic.
try switching to box mode
(tab key)
… much easier to see what’s actually going on…
clean box mode= clean model
messy box mode= messy model
as a general rule ANY “weird” behaviour in subd can be sorted in box mode…it’s almost always a problem with the underlying control polygon model. If stuff is crazy, crossed up and inside out, it will cause all sorts of hassle…
when in doubt box it out. 
(by hitting the tab key)
I am working in both modes, the result is the same. I noticed that if you have a subd-face with 5 points, like in the video where I start drawing the new edge, if that point is not aligned (literally on a line) with its neighboring points the edge either fails or it creates new point on top and the respective infinitesimal edge.
that is where you’d want to stitch any errant points to another to recapture correct topology flow-