I just had this exact same problem with my filters randomly deleting themselves in adblock plus 0.7.1. I did a search and this thread seems to be the only one I found dealing with this topic. I have no idea what caused this bug, so I know I couldn't intentionally reproduce it.
This is the first time this has happened and it occured randomly while I was using Firefox today. No specific site caused it (since I was just browsing my normal sites when I saw ads and pop-ups suddenly reappear).
I tried re-blocking some of the ads immediately after I saw the problem, but the filters weren't working. I would create a filter, see it in the main adblocked list, then go to block another... but when I clicked for the next filter, the list had deleted itself again. I just tried using the filter again and it seems to be behaving normally now (this about 10 minutes or so after the original problem, without having restarted the browser).
I am not using any subscription type filters - everything in the list were ones I added. I also checked, and I only have the newest version of adblock plus installed.
Okay, here are the system specs:
Windows XP Pro, (up to date, SP2)
Download Manager Tweak 0.7.1
Session Saver .2 dl * nightly 31
Adblock Plus 0.7.1
IE View 1.2.7
Bookmark All 1.2.1
All of my extensions are the most recent versions (according the the extension update manager).
I went and checked the patterns.ini file, and the most current one is only 1 kb. The backup files are: 10 kb for patterns-backup1.ini, and 8kb for patterns-backup3, 4, and 5. There is no backup2... not sure if that's relavent or not.
If you need any other information, just let me know... hopefully I've covered everything that might be useful.