Supposing my opponent is tapped out, has no floating mana, and has nothing on the battlefield capable of directly generating mana at instant speed (No Moxen or Loti, for example), or of untapping their lands, or otherwise being self-evidently relevant.

Is there any risk of the opponent acquiring new mana, or otherwise casting non-zero-mana spells? Are there any zero-mana instant-speed spells?

Can you think of any abilities, that could really subtly end up with mana available, but might not be obvious at first glance?

Basically, does "tapped out + no obviously relevant abilities on the battlefield" equal "I am guaranteed that I can play un-hindered?"

Ideally, it would be good to know the answer for any given format (Standard, Modern, etc.)