Excessive horizontal mouse scrolling On a MacBook Pro with OS X 10.9.1 (13B42) with both the trackpad and a Bluetooth "magic mouse" I observe an excessive amount of (especially) horizontal scrolling.

A trivial touch to the mouse (or touching the trackpad with two fingers) may result in dozens of (especially horizontal) scroll events. This is especially pronounced in Excel, Firefox, and Emacs (which notice horizontal scrolls) but not in Terminal and Outlook (which mostly ignore them).

Mouse tracking in system settings is set to the 3rd notch (out of 10).

It is possible to disable or reduce horizontal scroll events?
Yes, you can disable horizontal scroll events. I know this because unwanted horizontal scrolling with the Magic Mouse used to drive me insane, especially with Adobe products.

MagicPrefs, a "free application […] to improve the functionality and configuration options of the Apple Magic Mouse", indeed let me disable horizontal scrolling. It's somewhere in the "Magic Mouse" settings tab. Hope this helps.