OK, here's what I remember of a fic.
1) Harry Potter, (almost certainly) not a crossover
2) Harry is fed a potion that turns him into a girl
3) The person responsible for it gets into a massive amount of trouble and may or may not be essentially sentenced to become Harry's slave
4) Gred and Forge fake up having made another batch of the stuff and used it on each other (theirs is just a temporary transformation for funzies)
5) Pretty sure either Molly or Ginny was the potioneer.
1) Harry Potter, (almost certainly) not a crossover
2) Harry is fed a potion that turns him into a girl
3) The person responsible for it gets into a massive amount of trouble and may or may not be essentially sentenced to become Harry's slave
4) Gred and Forge fake up having made another batch of the stuff and used it on each other (theirs is just a temporary transformation for funzies)
5) Pretty sure either Molly or Ginny was the potioneer.