In "The Doof Side of the Moon": When Phineas and Ferb reach the moon with their skyscraper, you can see the U.S. flag that the astronauts planted on the moon on the far left side of the screen, and an abandoned lunar rover on the far right. Both are upside-down, of course, as they reached the bottom of the moon.
When Albert claims "no force on Earth" could make the building disappear, he was technically right as it gets moved by the moon, which is not a force of Earth.
Candace is not allowed to drive a car alone; in the US, a new driver must complete one year of supervised driving before they can drive unsupervised. Since Candace only has her learner's permit at the time, this implies she's still a student driver.
This is the first time Candace got grounded.
Candace decides to join Stacy at the lecture because she tells her there may be cute boys; however, Candace is dating Jeremy and talking about other cute boys might ruin their relationship.