On 2 separate occasions, the forward collision warning system activated. On both occasions I was under control of the vehicle with my foot on the brake approaching a vehicle that had slowed while entering into a round about...I was approximately 10 feet from the car in front going about 6 mph. All of a sudden several warning indicators activated (audible and visual) and then out of nowhere, all four wheels locked and my car came to a skidding/juddering stop. I immediately looked in rear view mirror fearing I was going to get rear-ended. The vehicle in front proceeded to move on and I tried to get myself together and get moving. I was shocked that this had happened as it was totally unnecessary. I was completely in control of the vehicle prior to the activation.