I don't think it was an accident. I hung on to a couple training manuals thinking I could sell them after the event was over only to discover that they were bound, unlike the other winter items.
It seems odd, as they usually mark items as limited time only if they are to be removed after the event, such as the Breen costume from last year. Also, you can still buy Q's Throne, of all thing.
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There's at least 1 other thread already covering this citing examples of this happening before. Did you look there for a possible answer?