I normally only seed one file at a time. My T Preferences are configured as:
Seeding with maximum of: 1 active transfers
I also tend to only download one file at a time. I've noticed that T tends to ignore the "one seed at a time" thing when an existing seeding torrent finishes up. So it'll go like this:
1. I'll queue up maybe 4 torrents. The first one will start, the others will be "Waiting to download".
2. Torrent #1 will finish downloading and start seeding. Torrent #2 starts downloading.
3. Torrent #2 finishes downloading, goes into "Waiting to seed" mode, Torrent #3 starts downloading.
4. Torrent #3 finishes downloading, goes into "Waiting to seed" mode, Torrent #4 starts downloading.
5. Torrent #4 finishes downloading, goes into "Waiting to seed" mode.
6. Torrent #1 finishes seeding. Torrents #2, #3 and #4 all start seeding at the same time, contrary to my settings.
Here is a log example.
2011-03-05 22:55:49 +1100 torrent.c:439 [Info] file-1: Seed ratio reached; pausing torrent
2011-03-05 22:55:49 +1100 torrent.c:1731 [Info] file-1: Pausing
2011-03-05 22:55:49 +1100 bencode.c:1716 [Info] Transmission: Saved "/path/to/etc....resume"
2011-03-05 22:55:49 +1100 Controller.m:1885 [Info] Transmission: restarting seed torrent in mac queue
2011-03-05 22:55:49 +1100 Torrent.m:318 [Info] file-2: restarting via startTransfer
2011-03-05 22:55:49 +1100 Controller.m:1885 [Info] Transmission: restarting seed torrent in mac queue
2011-03-05 22:55:49 +1100 Torrent.m:318 [Info] file-3: restarting via startTransfer
2011-03-05 22:55:49 +1100 Controller.m:1885 [Info] Transmission: restarting seed torrent in mac queue
2011-03-05 22:55:49 +1100 Torrent.m:318 [Info] file-4: restarting via startTransfer
file-3 and file-4 have basically jumped the seeding queue.
This is not a new issue. I've seen it in previous versions, certainly back as far as 2.1, so was not introduced in 2.22.