Any time you replace a key/value on a secure frame, you're liable to break it, so these lines are potentially problems:
Code:
ObjectiveTrackerFrame.collapsed = true
ObjectiveTrackerFrame.collapsed = nil
They may seem innocuous, but since reading/writing them may happen in a code path that can't handle taint, they may in fact break the buttons.
There are also other ways to taint the objective frame, apparently involving the world map (probably because the world map shows quest stuff now) but I haven't looked into it in great detail. For example, right now I'm not using any addons which touch the objective frame at all, and the buttons still don't work, with a random "action blocked" error if I try to click on them. I'm still using ExtraQuestButton, which I linked before, and have enabled the quest item ring in OPie for the occasional case where ExtraQuestButton picked the "wrong" item (eg. when the relevant areas for two items overlap).
At some point I plan to write a full replacement for the objectives tracker, since as far as I'm concerned the current objectives tracker is an unsalvagable disaster, but I have no ETA, and it likely won't support item buttons at all.
It's also worth pointing out that Blizzard could solve the problem very easily, by making the UseQuestLogSpecialItem only require a hardware event (eg. a click). I understand they don't want addons automatically using the items for you to complete quests, but I really don't see any serious potential for abuse if it just requires a click/keypress. It's a quest, not raid progression or tournament PvP.
