I've been using UIAutomation to produce screenshots with snapshot tool. It works great except one problem:
I can't tap on the correct button in UIAlert

The Instruments Automation tool produces this scrip code

var target = UIATarget.localTarget();

When you try running it, it will not tap "OK" button, but instead it will always tap default cancel button.

After struggling for a while I found that this behaviour is actually documented on Apple website. It says that in order to be able to control taping on UIAlert buttons you need to add this code.

UIATarget.onAlert = function onAlert(alert) {
  return true;

When you return true you take the full responsibility for correct actions handling of UIAlert and after that you can tap any button of UIAlert

Handling Alerts

When UI Automation encounters an alert during the execution of your script, it calls your alert handler, passing a reference to the UIAAlert object representing the alert. Your script should handle the alert appropriately and return a value of true, upon which normal script execution continues...

Read more at Apple's UI Automation JavaScript Documentation, "Handling Alerts" sections