Issue tracker:
http://forum.cockos.com/project.php?issueid=5478
FINALLY found the exact circumstances where this happens. Been bugging me for a while but hadn't sat down to figure it out.
1. Scrub audio using Ctrl + MMB
2. Let go of Ctrl but not MMB
3. Attempt to drag the edit cursor offscreen
Result: Sometimes it works fine (gets stopped at the edge), but most of the time it flies off into the distance, usually to some far away measure number, such as 1473925.
Note: it happens regardless of playback or not but its much more reliably recreated when playing back, I've noticed.
My first licecap, btw.