I am building a real world web app with Castle Project right now. So, the debugging is done for IE. And I encountered two different things of interest:
1. Javascript-based form validation as it's implemented for Castle Project does not work in Opera. Even more, pressing submit button in Opera causes a big stack trace of errors. Correct me, if I'm wrong, that's because IE's lack of standarts. I really 
dont know what's the problem is, because I'm not a tough javascript programmer.
2. IE automagically handles OPTION tag insertions into select lists. I have complex form, with a lot of selects, most of them have other selects as parents. E.g. City has District and Street as children. So I want to populate District and Street using ajax call invoked using onchange event hook of City select. I have two invocations for each one of the two. I pass District Select Id to the Ajax.Request object. And what I wanted to do was to simply get a lot of lines like

...
< value="district_id">district_name
...

by parsing my NVelocity template. And actually the code did return right list of options. But when it tried to insert them into District Select (with javascript using innerHTML property, I think. read about Prototype js library here) in IE 6 that resulted in messed tags (i use IE Developer Toolbar to see changes):

...
district_name<>< /option>< /option>< //option>
...

In Opera however, that didn't happen and it worked just as it was expected to. So after 2 hours of trying to understand what's going on, I found explanation to this. The thing is, MS does not allow option tag insertions without corresponding select tags. So, be concerned about this.