|Tip 259||Avoid Trackpad Confusion When a Mouse Is Used|
In my experience, Apple designs its portable Macs well and positions the trackpad to avoid accidental touching while typing. But if you’re using an external mouse, you might still find that you occasionally brush against it, causing the mouse pointer to skitter.
Fixing this is easy, but Apple has buried the setting you must alter. Open System Preferences (Apple menu→System Preferences), and click the Accessibility icon. Click the Mouse & Trackpad icon on the left, and then check Ignore Built-in Trackpad When Mouse or Wireless Trackpad Is Present.