Noticed this in a TON of games since the Final Fantasy set dropped. People are regularly casting spells without having enough mana to do so. I'm fairly new so at first I assumed I was missing something that gave them mana but since the FF sets dropped I've had it happen in like 5+ games, some of them running basically mirrored decks to mine so I know I wasn't misunderstanding a mechanic. Last game had someone with only basic lands, no extra mana abilities, no artifacts that add colorless mana spend 3 more mana than they had lands. (5 lands on the board, cast spells with a total value of 8). none of the spells they cast allowed them to play anything without paying its mana cost. The next turn they spent 7 mana with 6 available on the board, again with nothing letting them play cards for free. 3 extra mana is the highest I've seen this happen with.