To patch the client, the patcher downloads patch2.txt (via http) - this contains a list of numbers (in order, but not necessarily sequential) and patch files corresponding to each number.
The patcher stores the number of the last patch applied in patch.inf - it reads this, and if there are higher numbers in patch2.txt, it downloads those patches (via ftp), and applies them.
After applying all patches, it updates patch.inf
Sometimes, in circumstances that remain unclear, patch.inf gets set to 0 - when this happens, it will obviously start patching from the earliest date listed in patch2.txt - april 4, 2011.
When this happens, you need to IMMEDIATELY stop the patcher (if you let it apply any patches, you need to let it apply all the patches from 2011 to now, because the patch it just applied overwrote good data with old data - either that or reinstall). Then:
1. Open patch2.txt, and find the number of the most recent patch that you're 100% certain was applied.
2. Go to Ren's patch.inf generator http://choobs.org/stuff/patch.php enter that number, and download it.
3. Replace the patch.inf in your RO folder with the one you just downloaded.