Bug in 2.46: Intro.Char Anim book, pg 324, 'add' option not working

Hi. I’m working through the Action/NLA section of the book. Everything makes sense.
Refer to page 324, last line about ‘Add’. I expected my swinging bone to smoothly swing from back-to-front and side-to-side in parallel. (summing to a linear diagonal swing). However, it looks like the quaternions aren’t adding in a sensical way.
F-to-B movement is smooth but the S-to-S movement just snaps from extreme to extreme.
This seems to be a bug, see why, below.
I’m using Windows XP SP2, Blender version 2.46 official release.
I also just tried the same thing in the official 2.45 release, it works fine.
I went back and repeated the same (now that I know what I’m doing) actions in 2.46, same broken result. Saved the .blend file.
Then I downloaded build 619_blendersvn_rev15269, opened the saved file, still not working.
(schizo smiley: :yes::no:)