I had a FFN that was negative and I was having contractions, but they weren't labor contractions (no dilation or effacement). However my water broke within the next couple of days and then I did go into labor. She was born at 33 weeks. It can't predict rupture of membranes, which was my problem with both my pregnancies (my first was 36 weeks.)
I had a negative FFN 6 days before I delivered twins. Supposedly they are less accurate with twins.
I had one with my youngest son at 33 weeks which was negative too. His was accurate and he stayed in until 8 days before his due date. I knew what had happened before but it did help me relax a little that I was probably going to make it to 35 weeks which was our original goal after 32 wk and 29 wk deliveries.