As far as my testing goes, this bug seems to show up under specific conditions:
1) The loop selection starts at least about 17 minutes after the beginning of the project's time.
2) The user stops playback just before the edge of the loop.
3) There is a folder track which has a send to another track.
Nevertheless, these conditions tend to be easily satisfied in larger projects, so at least for me, this is a major show-stopper, since I tend to work with short loops a lot. When the loop is short and there are many CPU-intensive plugins, it is very easy to trigger the bug, and I've had tens of crashes because of it.
Steps to reproduce:
1) Open the attached project
2) Start playback
3) Stop playback close to the loop's edge, while monitoring memory use
Result: reaper's memory use starts to grow slowly, and will only stop if playback is resumed or possibly some other action is taken, such as moving the playback cursor. In this test case, the leak is not huge, however it can get to hundreds of megabytes per second with larger and more complex projects.
You can follow my gradual understanding of this bug in this thread:
http://forum.cockos.com/showthread.php?p=1409140
EDIT: this bug is only triggered with Anticipative FX enabled.
EDIT2: in order to reproduce this bug, you need to have the "Render ahead" value set on at least 200ms, and "Allow on tracks without FX" enabled.