I use bt to download huge files - several gig each. I burn DVDs with them. I download them to a special directory that does not get backed up.

Unfortunately, bt puts creates a temp directory in ~/Library/Application Support/BitTorrent where the files live while they're being downloaded. If the temp files are left lying around, they get backed up. Ouch! I didn't see that coming.

To be fair, bt will probably delete these files when the d/l is finished, but I didn't wait that long, so my backup got clobbered with gigantic temp files.