Is it the cyclecheck? That's normal, everyone gets that... I wrote a probably slightly incorrect explanation of this before, but here it is: What it means is this: Particles feed information to itself cyclically based on time, (e.g. their previous position at the frame before plus new information gives new position) so if you move backwards in the timeline, or if you jump to the first frame 1, you're screwing up this cyclical flow of information (nothing to worry about) and it's just warning you that it might not give you expected behaviour. That's my understanding based on what I think I remember learning. Hope that makes some sense.