Bloo's voice actor misread one of his lines in "Destination Imagination", which leads to this gem for older viewers.
Mr. Herriman: Oh, Heavens! What's happening?!
Bloo: You pissed him off, that's what's happening!
"Bloo's Brothers". Mac brings Bloo to show and tell one day. All of Mac's classmates think Bloo is awesome, so they all imagine weird clones of Bloo, only to leave them at Foster's. Bloo being Bloo, leads to this:
"Race For Your Life Mac and Bloo:" The titular buddies are racing each other to Foster's, but Bloo gets lost along the way. He seeks the help of a senior citizen sitting on a park bench, but it turns out the man is more than a little hard of hearing:
Bloo: Excuse me, could you tell me how to get to Foster's?
Old Man: Whazzat?
Bloo: Foster's! I need to get to Foster's!
Old Man: Flatsberg's, ya say?
Bloo: * getting annoyed* Foster's! Foster's Home for Imaginary Friends!!
Old Man: Flatsberg's Cone for Mackinary Hens?
Bloo:Foster's Home For Imaginary Friends!!!
Old Man: Flatsberg's Gnome Floor in Mackin' Harry's Pants?!
This is especially funny because Bloo's Sarcasm-Blind moment here is coming 3 episodes after he was specifically taught about sarcasm.
The Funny Bunny Song in "World Wide Wabbit".
This little gem, from Coco and Mac.
Coco: (In a sinister tone) Co-co, co-co co co.
Mac: Coco...I think if we did that, we'd go to jail.
"Cuckoo for Coco Cards": Bloo saying "Come ooooooooon" till people give him their cards.
"Goodbye to Bloo": The episode itself ended on a bittersweet note. The ending where basically the opening is reversed, reducing fans to tears, however there was Mood Whiplash, because beforehand there was a crowner when Cheese is revealed to have permanently moved into Foster's, followed by all the imaginary friends voicing their displeasure at this development.
In "Cookie Dough" when Bloo is listing the positive attributes of his and Mac's cookies he states that they're free of dioxins. When no one knows what they are, he gives a detailed description of them then boils it down to "They're bad for you."
When Bloo decides to sell merch relating to his cookies rather than the cookies themselves we get this exchange where a customer calls him out on it:
Girl: Mister, can I have a cookie please? Bloo: We don't sell cookies anymore. Girl: You're dumb.