Twice now, on two different torrents, the download has stopped at 99%, both times with one packet left to download. The connections still seem to be active, but it's just sitting there. The second torrent was 11.7 gigs, and it has downloaded all but 2 megs of it, but won't finish.

I just updated to the most recent version (2.81) but that didn't change the problem.

What am I doing wrong?

When you look at the peers tab of the torrent, do you any peers at 100% completed? If everyone else is 99% the torrent just isn't complete in the swarm.
If you do see other 100% peers but you are stuck at 99% I would try pausing the torrent, right clicking on it and choose 'local files', 'force check', let the torrent recheck the local files then start it again see if it completes.

Thanks for the response. I'll give that a try if the problem comes up again. I finally gave up on the torrent for this one; it was a full season of a tv show, and every file but one had finished, so I just cancelled the torrent and downloaded that one file elsewhere. It was just curious, because it happened to two torrents in a row, the same problem, which made me think I'd run up against some kind of paywall limitation in Tixati or something. Glad to know it's not that.

I have the same problem all season (11) from a TV show that stops at 99%. It seems to me that it only happens when I download from a specific website.

I've had this happen but when I play the video file, it plays fine to the end.  It doesn't go nuts or anything so if it didn't download everything 100%, I wouldn't know.  Most of the time when it's stuck at 99% forever, I just check the file or watch it anyway and It's always been fine.  I just chalk it up to some file weirdness.

The 99% situation is usually caused by a very small data corruption in torrent content (the corruption may not get noticed easily if the torrent piece size is less than 2MB). Leechers facing this problem, can either wait for the torrent to complete, or look for that content somewhere else. For seeders, this can be avoided by adding par2 files (using MultiPar for example) after finishing the download, then checking the torrents once every few months, or by using a system file that manages checking/correcting data, like ZFS or Btrfs.