A form of hidden track in which audio is recorded prior to the start of a CD. It's accessed by rewinding to before the start.

Aside from testing each of your CDs individually, what are the symptoms of negative tracks, eh?

• Read the CDs small print to see if it has a disclaimer along the lines of "CD doesn't fully conform to the Philips Red Book standard". This will usually be printed somewhere on the outer CD -- to warn consumers with older, un-compatible, CD players. One's that don't play this broken standard.
• The slightly more obvious song credits, and copyright, for unlisted songs.