Couple of problems that I've encountered:

When a user (1) has gone offline and another user (2) goes to a different page, the old chat window appears even though 1 has gone offline.

When, for example, you have three chat windows open and you navigate to another page, these windows re-appear, but if you have manually closed the middle window on the previous page the remaining windows don't recognise this and there is a space between the two remaining windows.

Cheers,
Crom