The mp3 I was trying to transfer had name tag "2 + 2 = 5", artist tag "Radiohead", album tag "Hail to the Thief", and genre tag "Alternative Pop/Rock". There's got to be some kind of a bug if MC thinks these are seperate songs (sorry for not making that clear).
I just tried transfering a completely new song and got a 132 error, without any of that strange trying-to-transfer-genres stuff. I think I've stopped getting the 139s and 2560s.
Given the many different possible ways of treating the handheld (built-in MC autodetection capabilities, plug-ins, WMDM, etc.), it's possible I'm not using the right sort of method or configuration. As it is now, when I plug in the Zen via USB a node appears below "Drives & Devices" called "Creative Zen Touch (Joshua Horowitz)". Underneath this node are "ALBUMS", "ARTISTS", "GENRES", and "PLAY LISTS". The only handheld plug-ins I have installed are Portable Drives and WMDM.