Owing to the limited precision available on today's GPUs, Realtime Mandelbrot doesn't allow very deep zooms. (The greatest zoom factor is 25000, corresponding to an image a few million pixels wide and high.)
For a future update, I plan to switch to the CPU for greater zoom factors, instead of just limiting the zoom. The will come at the expense of a slower display updates (for zooms beyond the switch-over point).
Chime in here with your opinion as how to important that feature would be for you. If many people want it, it will happen sooner than otherwise.