Medusa seems to have a solid grasp of 21st century idiom and slang, despite having been locked up since (presumably) ancient Grecian times.
Not exactly a goof, but...we're hit over the head twice with the point that "magic always has a price." But Circe never seems to pay a price. Neither does Zatanna except when she's trying to buy a magic ingredient.
So, what the heck are Zatanna's powers? Sometimes she makes backward invocations, sometimes she just waves her fingers, sometimes she needs a spell component (the chimera scale), and she has at least one magical artifact (her VCR-crystal ball). If she can enlarge her hand and grab someone then pull them back up into her hat without saying anything, which is pretty powerful mojo, why does she need the reverse-magic stuff sometimes?
Why does Circe need a crowbar? She can transform both animate and inanimate matter (she creates walls out of a stone wall and turns a batarang into doves).
The Batman doesn't want to bring in a sorcerer like Dr. Fate to keep the whole thing (somewhat inexplicably) secret. But then he brings in (or lets B'wana Beast bring in)...the Crimson Avenger? To go door-to-door looking for a pig? Other then as a cheap sight gag, it doesn't really make much sense to bring in the Crimson Avenger but exclude Dr. Fate, who might be able to actually do something to restore Wonder Woman or find her in pig form much more quickly via magic.
Circe is powerful enough to transform Wonder Woman's armor, but not her bracelets. However, they were both created by Hephaestus, so she should be powerful enough to transform both... or neither.
The stockyard worker pulls a gun from the locker - it appears to be a two-barrel side-by-side shotgun. Shotguns fire buckshot in a wide spread. However, when he shoots Wonder Woman she deflects the shot as if it was a bullet.
Circe: (as Batman sings) Beautiful.
Zatanna: And you'll keep your word?
Circe: A deal's a deal. (short pause) He can stop now.
Zatanna: Not on your life.
Circe: You're both beginning to remind me of Sisyphus after a hard day pushing his boulder up a mountain. But me, I could keep this up all night.
Circe: Insolent trickster. You dare to strike... (gets hit) You dare to stri... (gets hit again) You dare to... (gets wrapped in a tablecloth)...Quit it! Oh no... (gets hit by a piano)
Circe: Thank you, thank you. You've all been so kind to me tonight. You know, my cousins the Sirens never thought I could sing. Really! They thought they were all bad with their 'Charm the Sailors' routine. But I'm showing them, aren't I? (dead silence)
(Circe glares - applause)
Oh thank you! I love you all! Are there any requests?
Zatanna: Selbat ekirts ecric!
Circe: I'm not sure I know that one...
Medusa: When you see Circe, tell her I want my curling iron back!
Themis: Thank you, Medusa. And know that you've earned 300 years off your sentence.
Medusa: Freedom in 4010--ring-a-ding-ding.
Crimson Avenger: Excuse me, sir--I'm looking for a pig.
Man: Gladys--it's for you!
Themis: You were Circe's cell mate, weren't you?
Medusa: C-Girl and me did some time together back in the Pit of Eternal Torment. That's where they hang you by your ankles and weasels come to eat your fingers each night. They grow back the next day, but trust me--it gets old real quick.
Red Tornado: Sioouuuueee!
B'wana Beast: Yo! Wonder Pig!
B'wana Beast: B'wana Beast – how you doing?
Zatanna: My legs are fine, as is the rest of me. Up here?
B'wana Beast: Wow, she's got the fire of a cheetah in her. Grrr.
Zatanna: Why are you doing this to me...?
Wonder Woman: Circe! Be careful, she's...
Batman: Yeah, I've read The Odyssey.
Wonder Woman: No, no dating for the Batman. It might cut into your brooding time.
Batman: One - dating within the team always leads to disaster. Two - you're a princess from a society of immortal warriors, I'm a rich kid with issues - lots of issues.
Wonder Woman: Don't you ever wish you were down there?
Batman: I'm down there all I need to be.
Batman: Patience, Princess--Intergang moves in mysterious ways.
Zatanna: Is there anything you can't do?
Batman: There's one thing I've never been very good at-–saying thank you.
According to the commentary, there was a scene planned where the Joker plans an ambush on Batman. He calls it off after seeing that Batman is talking to a pig (Wonder Woman).
While writing this episode, Paul Dini would meet and later marry an illusionist. Paul's favorite show character was Zatanna, who was used in this episode.
Zatanna continues to use her backwards talking magical spells. For example to get her money back from the magic store owner she declares "dnufer". However whoever does the captioning is apparently unaware of the nature of her powers since they render her lines as nonsense words. However a viewer can translate all of her spells by listening closely.
Character bios: Zatanna , Circe, B'wana Beast, Red Tornado
Peter Onorati (B'wana Beast, and seemingly channeling Joey Tribbiani on Friends, right down to his ability to deal with farm animals) has played a Justice League member before - he was Warhawk in the Justice League of the future seen in the Batman Beyond episode "The Call".
We see the Iceberg Lounge next door to the museum at the beginning - presumably this is the Penguin's restaurant, as established in later episodes of the Batman Animated series. That and the reddish skies would place most of the episode in Gotham City (unless he's opening franchises).
Red Tornado, Elongated Man, and the Crimson Avenger all briefly appear - the Red Tornado and Crimson Avenger have brief bits of dialogue but no voice actors are credited for their parts.
Lulu's Back In Town - written by Al Dubin and Harry Warren. Am I Blue? - written by Grant Clarke and Harry Akst.
The Intergang from various Superman: The Animated Series episodes are mentioned but not seen in this episode.
Jennifer Hale takes on the role of Zatanna, played by Julie Brown in the Batman Animated episode "Zatanna". There are several allusions here to the relationship they had in that episode.
The Underworld Unleashed
The blindfolded goddess with the sword and scales that Batman and Zatanna meet in the Underworld likely represents Themis, one of the Greek Titans, and the Goddess of Justice. The boatman ferrying Medusa is probably Charon, who would carry the dead across the river Styx to the Underworld. He would only carry those who paid a fare, that's why Batman gives him the coins.
Man: Gladys, its for you!
A reference to Bewitched where the Stephens' next door neighbors are the Kravitzs. The long-suffering husband, Abner, inevitably just misses his wife Gladys' attempts to show him some evidence of Samantha's witchcraft she's just witnessed, and assumes she's nuts. The husband here looks a bit like Abner Kravitz (actor George Tobias).
Circe: You're both beginning to remind me of Sisyphus
In Greek mythology, Sisyphus attempted to evade death, first by imprisoning Hades and later by claiming his body was unburied and he should go back to assure its funeral. For his crimes he was sentenced by Hades to push a boulder up a mountain for eternity - each time just before getting to the top it would tumble to the bottom and he'd have to start again.
Zatanna: C'mon Bugs...where are you?
Subtle allusion to Bugs Bunny, the WB animated icon.
Themis: Don't look at her directly.
The reason that Medusa must not be looked at is because she turns people to stone that look her in the eyes. The statues outside the ampitheater are not works of art, but victims.
Circe: That trick always went over big in Egypt.
When Moses confronts the King of Eygpt, he turns his staff into a snake. Here Circe turns her staff into a snake and alludes to the Biblical incident.
When Circe is hit by the piano her "ruby red" slippers and legs flatten and curl up, just like the Wicked Witch of the East does under Dorothy's house in The Wizard of Oz.
Factory Worker: That'll do, pig
Using the phrase from the movie Babe.
Themis: Uh... There's a small service charge.
After Batman and Zatanna interrogate Medusa, the boatman (Charon?) asks for a "service charge." Batman reaches into his utility belt and pulls out two coins. In ancient Greece it was believed that when a person died they had to be put on the pyre or buried with two coins covering their eyes. This would act as payment to the boatman so that they may cross the River Styx to the land of Hades. If they did not have the two coins, they would wander the shores of the River Styx blind, deaf, and mute.
Medusa resembles old-time movie star Bette Davis. According to the DVD commentary, Medusa's appearance was based on actress Tullulah Bankhead, and if they hadn't already recorded her dialogue before animating it, the producers would have instructed voice actress Laraine Newman to play the character to match.