The amount of bad data on last.fm and their lack of interest in cleaning it up makes me sad. You can see a small example just by looking at the current top songs: http://www.last.fm/charts/track

Morning Mr Magpie is in position 8 instead of position 4 where it should be, because 8105 people scrobbled Morning Mr. Magpie with a dot. Tip of the iceberg.

Really egregious are the albums with no scrobbles because every track on the album has "(Album Version)" or some such affixed to its name.

Once on a Pandora station they played me a track by a band called Burial, instead of the dubstep artist Burial I had added to the station. So I sent them an email and they thanked me and told me the issue was very important to them and they cleaned their data. Last.fm doesn't bother, even when it causes their top-20 charts to be completely wrong.