With OS X 10.9 Apple takes security more seriously than it ever has. In this regard, Apple has added an additional level of security for apps that access the Accessibility API via "System Events". Here's how that new level of security may affect the way you use some AppleScripts downloaded from this site.
First of all: Some of my script applets (the ones with user interfaces) cannot run effectively while iTunes is in full screen mode. This is because only one of them, iTunes or the script, can be frontmost. So, when these scripts are launched they will detect and alert you if iTunes is in full screen mode.
To do this the script targets "System Events" to check for an accessibility property of the iTunes user interface indicating whether or not full screen is on. "System Events" is in the purview of Accessibility. If you've ever used GUI Scriptingto script key codes for exampleyou've had to set "Enable access for assistive devices" in the Accessibility pane of System Preferences. Well, as of OS X 10.9, this is no longer a global setting and must be set on a per-app basis.