So, let's say I have a Brazen Collector and a Firebending Student both attacking, I get 2 floating Red Mana. Let's say I activate an ability that uses one mana, and then pass to main. I was then counting on still having the one floating Red Mana from the Brazen Collector available to cast a spell during my Second Main Phase, but guess what? Arena used that floating Red Mana, and let the other (More Temporary) Firebending Mana expire when the Combat Phase ended. I was going to use that Red Mana to help pay for a board wipe, but I couldn't and I lost. Sad.