Hi, This sometimes happens to me because I have my thumb over the command key while I scroll, especially if i scroll just before using command-Tab. My Logitech my mouse wheel will continue moving after I have lifted my finger.

Is that what's happening to you?

Probably not. My "zoom" problem seems to occur simply by touching my mouse. I then hold down the Command key and scroll with the mouse to reduce the size, but then when I load a different web page, the new page is tiny, and I have to hold the Command keg again and scroll up to return to a normal size. Confusing! Thanks

Could you try using Command+0 to reset the zoom level.

(That would be for the "reflowing" zoom where contents is enlarged and re-flowed withing the window, and not the "pinch" zoom which magnifies part of the page.)