So was just reading through the secret histories and got to Practice, and I realized something that made me double-check the base rules of the game:
"Once a player begins to buy cards and/or Events, they may not play any more Treasures."
In most other situations where you draw cards in your buy phase (such as playing a Market with Capitalism) it happens while playing treasures, so any treasures drawn can still be played. However if you buy Practice, you have now started using your buys, so you are not allowed to play treasures you may draw via the action doubled by Practice.
This may seem obvious to veteran players, but I'm betting a lot of casual players are going to see this and assume they can "Practice" a Smithy and then (assuming they have another buy) play any treasures they draw to buy something bigger.
Edit: And then I just got to Sea Trade in the secret histories, and realized that its another case of drawing cards in your buy phase via event, and could use this same clarification.