1 Harry: {skeleton} This is not good, Hermione.
1 Hermione: Sorry, Harry.
2 Harry: Quick, cast a reversal charm to undo the shape change.
2 Hermione: Okay.
3 Hermione: {waves her magic wand} Morpholio batrachus!
4 Hermione: Oh Harry!
4 Harry: {now a toad} Ribbit. I think you meant backtrackus... Ribbit.

2012-01-26 Rerun commentary: Interestingly, if you search for the word batrachus, you'll probably come up with Clarias batrachus, the walking catfish. I didn't realise that until just now when I started writing this rerun commentary. There's also Asterophysus batrachus, another species of catfish. And as a genus name, Batrachus refers to some of the fish of the Batrachoididae family, commonly known as toadfish.

Now Harry has obviously not been turned into a catfish here. Nor a toadfish.

Perhaps I should have used "batrachos", which leads more directly to the root meaning of this word, being Greek for "frog". (The fish are presumably so named because they are frog-like in some sense. "Toadfish" confirms the link for those ones.)