Using a recent nightly, TortoiseSVN 1.6.99, Build 21076 - 64 Bit -dev, 2011/03/28 20:29:13, I have a problem with temporary files disappearing.
The command I run is TortoiseProc /command:diff /path:"File.txt" /hwnd:123456 .
In SVNDiff, line 199, a temporary file for the base is created. Then, in line 200, it is filled, and the date stamp is changed to the time of the file’s last commit. For this file, that time is two months ago. This means that it counts as an old temp file and is deleted when TProc exits, and TMerge does not find it.