If this happens only when you're using Safari, it's due to a clever zooming system that is prone to zoom in unintendedly (at least for me). Two quick, light clicks at the front of the Magic Mouse will zoom you back out or, if repeated, back in again. It's useful for reading newspapers and such--the zoom system seems to detect the widths of columns of text.
Yes, I knoow how to zoom out. Thanks. I didn't quite realize how sensitive zooming in is. I am not even touching the surface of the magic mouse just moving it forward, and that's when it zooms. It does not do this consistently.
Dave thanks so much for you help, and reply. It has provdied with insight about how sensitive is it with Safari.
FWIW, this happens in Firefox as well.
The solution is to turn off the "smart zoom" feature in the mouse control panel.
The Magic Mouse is highly sensitive; it will sense your finger before it actually touches the surface of the device. Get very close, with the tiniest bit of a wobble, and it thinks you just gave it a "double tap".
Apple could maybe fix this by updating the control panel, with a minimum delay between "taps".