True, but I think the number in question was the peak flow, not the flow at any one point. I have been receiving the same message and I believe it has something to do with Hydraflow screwing up whether or not things should be controlled by inflow or outflow.
Although I believe I have diagnosed the problem, I have yet to find a solution and am curious if anybody else has found one.
Usually this happens with a small flow & large weir. A weir is usually the highest , ie. 100 year discharge. With the flow dependant on H^1.5 you can go from 1 cfs discharge to 10 cfs discharge with a 0.01 ft increase in head.
Set your weir length for 100 year storm down to something very small, such as 0.5 or 1 ft weir length and see if you still get the error.