For a new 6-players campaign were I rush the players through the first four levels (1 session = 1 level), the sorcerer and the wizard players quickly saw that I'm the kind of DM that allows everything as long as it's fun for everybody.
Those two players like to optimize but only start at it, so after the first session they ask me about a few spells that are OP. Of course, I mention Silvery Barbs. That hit something because I saw a lot of discussion between them on our Slack, but I left them theorycrafting as they wish.
Now after the third session, they come to me and ask me whether I take any issue if they both selected Silvery Barbs as one of their swap spell for the next level up. My answer was simple: "no, but then Silvery Barbs is fair game for my monsters as well". They were a bit surprised, but I saw them thinking. I totally did not expect what they answered: one of them said that then it would be less fun for the other players if my monsters had Silvery Barbs, and the other immediately agreed.
And no Silvery Barbs was put on a spell list!
I love my players :D