This only happens if you have one track in your playlist set as a beat mixed overlay:

I was starting a new mix today and I started try to loop a drum beat from one track using it as an overlay. However, it started speeding up the track and playing each drum sound in reverse. It sounds like it gets each sample and plays it backwards. I checked it on my old computer too and the same thing happened!! I tried it with several different tracks, ranging from 128kbps - 320kpbs CBR Mp3 and WMA files. Does this happen to anyone else?

I'm not sure that it is playing backwards, but I definitely get weird behaviour like this when trying to use an overlay. I never tried to isolate it by the number of tracks on the screen .. I'll try that later this afternoon.