When trying to add a bookmark or remove it (in the conference browser window), Spark freezes for a while or even disconnects and reconnects.
After removing all the bookmarks that were stuck (though removed via Bookmarks plugin on Openfire, but still showing up in the client), freezing went away. But when using 2.7.7 there were no freezing at all. So after update to Smack 4 Spark is now more unstable when dealing with such cases.
After bookmarking and remove a bookmark multiple times it stops showing the bookmark icon near a room and pressing Remove bookmark does nothing. If you try to log out it takes longer and is not properly logging you out.