From Second Life Wiki
Feature Design Document
Discussion for future improvements
Relationship to other features
List of features that need to be tested when this feature changes, and why.
Region crossing - After you cross a region in a vehicle are you and the other passenger still sitting and playing the correct animation? What if you're not allowed into that region?
Attachments - Your attachment is a child of your avatar which is in turn a child of the thing you are sitting on. Does the attachment still work correctly?
Teleporting - What happens if you teleport while sitting?
Camera control - Does camera control work well while sitting?
Scripted permissions - You automatically grant certain scripted permissions when you sit on something.