Even if there were problems that were solved by such collective action, you should not create plans that rely on things like that happening (by definition, you cannot create a spontaneous action). Your plans should not rely on the problem having to solve itself. Edit: unless the type of spontaneous collective action you need is known to happen often or the problem you want to solve is of the type that are known to often solve themselves.

Actions of the crowd during the fall of the Berlin Wall seems to be an example of an event that fits the description, as it wasn't centrally organized, many people simply tried to make use of opportunity that suddenly appeared due to actions of East German government and other circumstances.

If the words are interpreted broadly enough then the non-existence of a free online encyclopedia is an example of a problem that was solved in such a way.

Have there ever been no problems actually solved by massive, broad-based, spontaneous, decentralized collective action?

The defeat of the Evil Empire, aka the Union of the Soviet Socialist Republics :-D