While Mac OS 9 had an Apple-developed and sanctioned system for extendibility (the Extensions and Control Panel folders), no such add-on architecture exists in Mac OS X. As such, developers have taken to offering "haxies," retrofit modifications that can cause severe troubleshooting problems without the user ever realizing the cause.
Many of these add-ons fall under the APE (Application Enhancer) umbrella. Application Enhancer is a utility which - according to its own documentation - a system "which allows for 3rd party modules to modify and enhance the way applications behave and operate."
The documentation goes on to note "Application Enhancers work on an application level, therefore they do not affect the stability of the underlying system. The technology behind Application Enhancer system has been in research and development for more than 1.5 years now, and many Unsanity haxies (including WindowShade X, FruitMenu, Silk and others) are now using it. The software development kit for developing modules is also available on the Application Enhancer home page."
Unfortunately, we have known since the early days of Mac OS X that individual applications - particularly those that make any modifications to the System folder - can certainly affect the stability and performance of other applications, including the Finder.
Many troubleshooting issues received in the MacFixIt inbox that are at first attributed to a Mac OS X incremental upgrade or a disk permissions problem are actually easily resolved by temporarily removing Application Enhancer and restarting. Though APE itself has rarely been the culprit, individual haxies have been over and over again.
A system can be completely problem-free with multiple installed and active haxies. But if you are experiencing inexplicable glitches in the Finder, disable your haxies as a troubleshooting step.