"Nobody expects the Spanish Inquisition" - which therefore counts as spontaneous.

There is no reason whatsoever why a group activity organised in advance shouldn't be seen as spontaneous. If a bunch of friends decided to play football in the park every Sunday morning, I can't see how anybody would say that it wasn't a spontaneous activity.

From the late Latin root sponte "of one's own accord". It seems Civil Servants these days aren't just incompetant, they are illiterate.