I don't know if it can really be described as a bug, but oneswarm places thousands of files in my %temp% directory (windows).
Mostly thumbnails for the videos, sometimes also empty .xml files and persisted javabeans
<?xml version="1.0" encoding="UTF-8"?>
<java version="1.7.0-ea" class="java.beans.XMLDecoder">
<array class="edu.washington.cs.oneswarm.f2f.friends.FriendBean" length="15">
It's kind of a pain in the butt to have all of those files in there as it makes it virtually implossible to look for other temporary files