Some users have reported the error "Your sound card is either busy or not present. Try restarting the application" when trying to use the Last.fm client on Windows 7, although there are no problems with their sound cards.
If you're getting this error, check if you have the client pinned to the taskbar and if yes, try unpinning it from there and opening it from somewhere else instead (e.g. the start menu).
"Track set in directory not to scrobble" error when using iTunes
If you get this error when scrobbling what you play with iTunes, you'll need check if the program folder the Last.fm client is installed in (normally this is C: \Program Files\Last.fm) is set to not scrobble, and if yes, set it to scrobble as well. This problem has been occurring since the iTunes update to version 8.2.
Note that if you set the Last.fm program folder to scrobble, it will scrobble music from all your folders, including directories you have set not to scrobble.
Compatibility with foobar 1.0
If you update to foobar2000 1.0, you may need to re-install the Last.fm client's plugin for foobar2000 0.9.4, which works with v1.0 as well. In the Last.fm client, go to Tools > Get Plugin... and add foobar and download the 0.9.4 plugin in the following steps.
Scrobbling from an Ipod touch may give you exaggerated scrobbles.
There is a knownissue with ipod touch in that play counts are massively exaggerated in itunes. Hopefully Apple will address the issue in a forthcoming update.
Last.fm Scrobbler: Known Issues and Common Problems