According to the phone bill Homer receives with Bart's long distance charges the calls were made between 8:45 and 10:15 am. Bart would have had to made the calls in the afternoon, as after he hangs up on the six hour call to Australia it is nighttime in Springfield.
When Bart decides to take his booting at the American embassy, the green hoses on the toilet -- that force the water to drain counter-clockwise -- are missing.
The main premise, which is that the "Coriolis effect" causes all drains in a certain hemisphere to rotate a certain way, is in fact not true. The Coriolis force is too weak to cause any visible effect, and the water can rotate in any direction anywhere on Earth. The direction is primarily determined by the shape of the vessel and the way it was filled. Ironically, the "hemisphere determination" urban legend is still widely believed because of its use in several fictional works including this very cartoon.
The invasive bull frogs are seen eating the vegetation. American Bull Frogs, like nearly all frogs, are strictly carnivorous.
The guard at the US embassy says that it's considered American soil. However, foreign embassies are not the sovereign territory of their nations, nor are they above the laws of the host country. The reason for this misconception is that citizens of the latter may not enter them without permission, and that if the authorities suspect that illicit activities are being conducted inside an embassy, they have to wait for suspects to leave before arresting them.
Homer asks for a big beer and the bar tender puts a giant can of Fosters lager on the counter. While it's possible for the bar to stock Fosters, it is unlikely as it's not widely available and is primarily an export.
Lisa plays a didgeridoo with ease. Didgeridoos are actually very difficult to play.
When Homer is looking over the phone bill listing the Southern Hemisphere locations Bart called, Burkina Faso is listed; since Burkina Faso is completely within the Northern Hemisphere, it would have done Bart no good to call there. However, Bart didn't even know what the Southern Hemisphere is until Lisa told him about it.
One of the jokes is that the bullfrog Bart brings to Australia reproduces and they spread causing major damage. However, Bart brought only one bullfrog so how could it have reproduced? Answer: by cross-breeding with a local Australian frog of the same family (Ranidae). Cross-breeding of different genera in the same family is improbable, but theoretically possible, in real life (e.g. cattle and buffalo have made "beefaloes" and "cattaloes"), and even more likely in a comedy of The Simpsons' nature.
When Tobias goes to check with his neighbors about the toilets, he goes on a six-hour bike ride to get to the nearest house. Later on, however, Bruno explains the problem to his neighbor Gus, whose pig farm is right next to the Dundridge home.
When Tobias is confronted about Bart's phone call, Tobias's father refers to it as a "collect call" (American terminology). Australians actually refer to this type of phone call as a "reverse-charge call".
The Australian teenager calls bullfrogs "reptiles". Frogs are amphibians.
Bruno and Marge both refer to the cost of the call as $900. It's never specified whether this is US dollars or Australian dollars.