Notice: This happened when updating from VLC 2.0.7 to the latest version, using Sparkle. I have been able to reproduce it with VLC, but not to test with any other software (everything else is up-to-date)
If download fails because of (eg) a network problem, but the computer is still connected to the (here, Wifi) network, Sparkle, instead of reporting that the download hasn't been completed, fails with a message warning about digital signature (“The update is improperly signed”). If the wifi gets manually disconnected, a proper message appears, warning there was a problem with the download.
OSX Lion 10.7.5
Btw, this message isn't translated to French. It should be : “La signature numérique de la mise à jour est incorrecte”
Localize "The update is improperly signed."