The song "When we didn't get along" can be very depressing.
Some mild rude humor.
In "Out To Launch", Candace enters a decaying orbit, close to crashing. Intense music plays. This is very frightening, however Ferb saves her.
The last day of summer episode can be emotional and frightening.
In "Phineas and Ferb's Quantum Boogaloo", seeing the whole future completely dismantled in the law of Doofenshmirtz, and the past video broadcast by Doofenshmirtz Evil Inc. is frightening to children.
Dan crying at the end of the "Act Your Age" episode is very sad.
After being offered a "Follow-up single", Phineas gets mad at the director. Though this is unlike him, this may be unsettling for young children and Phineas fans. Played for laughs though.
In "Mission Marvel", Baljeet is turned into a Hulk monster by Candace in an accident. After he runs, Phineas thoughtlessly rises his anger at Candace, causing them to have a very serious fall out, causing Candace to storm out slowly in a very sad manner. He tears up her S.H.E.D. card. This is unlike Phineas, and can upset children and Phineas lovers. However at the battle scene, Phineas feels sorry, and says sorry to Candace, and tapes the S.H.E.D. card back together.
Summer Belongs To You: The scene where Phineas yells "GET ON THE TRIKE!" at Candace may unsettle younger kids and Phineas lovers, since anger is NEVER usual for Phineas, but may be played for laughter.
The phinedroids and ferbots can be a little frightening, when they're cornering Phineas and Ferb and going hot-wired.
In one episode Candace enters a model search for a cosmetics company however she is told that she is too short and does not qualify and is shown crying because of it. This may be upsetting to some viewers who have struggled with low self esteem or body dysmorphia.