general to the group: in spin training, what seems to have been been
the most popular way to screw up? i understand the drill and i'm having
trouble picturing how to mess up that badly in the typical training
cessana starting at maybe 2500' agl.

As someone else mentioned, using aileron instead of rudder.

Admittedly, I almost did it on my first ever spin while
training....it's instinctive, and it's a *hard* habit to break,
especially when you are at such an unusual attitude after the wing
drops and the nose suddenly is pointed towards the ground. :-)

I caught myself in mid-movement of the yoke, and corrected ailerons
back to neutral for the rest of the recovery, but the instinct to
correct with aileron before the plane is actually flying again, and
not stalled, is overwhelming.

On the first one or two spins, there is definately an "oh crap" factor
that causes your hands to override what your head is telling you is
the "right thing" to do.

My instructor suggested that I "Wedge" my elbow somewhere on the door
to prevent me from physically using aileron untill I got past the
actual spin, and have recovered from it. That worked for me, although
after the 3rd spin (I think we did 5 or 6 in the lesson plan) I had it
pretty much down, and really didn't need to do that any longer.

As for 2500 AGL you quoted, that's wayyyyy low.. Our training area
is about 650' asl, and we were climbing through to 4500' before each
spin... It was a comfortable height, and even after recovery from 2
or 3 turns in the spin, I never felt uncomfortably low.