The finder docs mention iterating over an FTP stream, like so:

$finder->in('ftp://example.com/pub/');

Turns out, this does not work. It errors out with:

[ErrorException]
Warning: FilesystemIterator::rewind(): stream does not support seeking in foo.php line 42

Other stream wrappers may be affected as well.

Looks like @krachot already mentioned this somewhere a year ago.