The one thing /u/__crackers__ left out is that all pieces do not download equally quickly. Such is the nature of a P2P network. It is not uncommon for a torrent to be stuck on 99.9%, waiting on that one nebulous piece to download.

Often the pieces that take the longest to download are from clearly less popular files. Download a TV series, and you'll notice that select seasons will download in minutes (usually the more recent ones), while others might take hours.

Also, sometimes the file you want out of a torrent is stupidly small, and the torrent old/poorly seeded. This has happened to me a handful of times. I still have to wait to download megabytes, sometimes at just a handful of kb/s, in order to get the little .txt file that contained a legally transferable and completely legit license key.