So... this was today's "trick" and while Pottermore has been so far a bit underwhelming in it's event, this tid-bit has prompted more discussion than the previous announcement of new spells. As such, it's time for speculation and analysis after the break!
This was going to be EXCITING. For the first time they were engaging with fans and users of the website on a greater level than just posting news articles on their Facebook Page or the Pottermore Insider. This was the kind of engagement or activities to keep interests piqued between chapter and book releases that members have been asking for almost three years. Pottermore even made a Facebook Event that included a poll which was another first for them. Speculation abounded for days about what these potential tricks or treats could be.
Since Pottermore is doing an event, thought, why not vote on your opinion. You can see it right to the side. If you haven't seen the event or signed up for it, click here.