There appears to be some "favorites are broken" discussion in the user forums as well:
http://getsatisfaction.com/twitter/topics/favorites_not_appearing_in_the_right_date_time_chronological_order_bug_some_pages_blank And further work suggests that timing is playing some to a large role here. Many of the items that could not be favorited 12-24 hours ago were successfully favorited just now. I have seen that delay several times in the last week. Eventually things that could not be favorited become favorite-able again. Most users would have long since moved on by then, however.
