Rendering error: Large rectangles of solid color in rendering

When rendering a semi-deep zoom image on the regular Mandelbrot set, the final image produced has large rectangles of solid color covering the minibrot. I assume this has something to do with petrabution?
The bug seems to vary in intensity...

The bug seems to vary wildly from render to render...

Those look like artifacts you get when you have periodicity checking turned on but it's IDing escaping points as periodic, except that they should have the inside color. Not sure what your are doing with layers here, but that might be it. Try turning it off and let us know if it helps.