I confirmed this behavior by creating a file with the .ffs_tmp extension on a source location for a mirror and it acted the same way. This is intended behavior
(Which would then also likely mean that something else is going on in that other thread.)
Maybe it's a race (not exactly the right word, but...) condition.
An existing x.ffs_tmp wants to be create a file named x.ffs_tmp, but it already exists.
So you would think it would create a file a file named x.ffs_tmp.ffs_tmp, but it only wants the basename, so we're back to "x", which then would create a x.ffs_tmp file, which already exists... heh.
" .ffs_tmp" files are a special case: They "belong" to FreeFileSync. If a new run finds them remnant from a previous sync, they will be unconditionally and permanently be deleted as a cleanup. See "fail-safe file copy".
