This has happened when doing a 3rd gear pull. Here are 2 datalogs of it. The AFR is at the bottom in red. Ther could be 2 causes of this: for one, the car is rich as hell. You can see right before it goes to 17:1+ that I'm at 10.7:1, probably lower, but I doubt the AEM can read that low. So, there is a chance that too much raw fuel is getting on the sensor, fouling it, and causing it to go instantly.
The other cause could simply mean it's just going bad. Looking at my duty cycle, it really isn't that high, though I am very close to maxing the injectors out. Anything over 100% isn't possible, but I'm willing to say that over a few boost spikes, the injectors have hit 100% DC. Maybe that is what is really happening behind the falsly calculated scenes and they are just dumping in as much fuel as they can.
http://forums.plastikracing.net/files/graph1_144.png
http://forums.plastikracing.net/files/graph2_136.png