From my experience with some programs I am developing in C, I think the cause of this kind of error might be a variable that got an inf, -inf or nan value (IEEE specifications for floating numbers). Once a variable has got such a value, no operation (addition, multiplication, etc.) will change it, and this will be transmitted to other variables. Of course, this does not explain how this happened in the first place. What I don't know is what happens if you run fractint using exactly the same parameters once more. In case the symptom appears in a haphazard way, it might indicate an uninitialized variable is taking values left in the memory by other programs. <<
Good point (no pun intended). Certain pixels do cause the bug. Usually, it is repeatable. One can even pan the image up, left, etc., and the same pixel in the fractal image will still cause the bug. I've noticed in a few cases, it occurs when the showdot hits the edge of a lake, but this may be a coincidence. But occasionally - it is not repeatable! These images often use function=exp/exp/exp which could possibly run into inf or -inf.