The <sess:existsattribtue> always executes both tags whether or not the attribute exists. It failed on my code first, so I tried the example provided with the documentation and it still failed. I'm working with Tomcat-4.0b5 on Windows 2000 (Service Pack 1)with java-1.3.1 installed. I also tested it with Tomcat-3.3m4 and it produced the same result with both tags executing instead of just the one of them. To reproduce the problem, just cut and paste the example of the tag from the docs and test.
The session taglib recently had a face lift and was tested on the latest Tomcat 4 build from CVS. It worked fine.