Aquaman: There you are! I'd like to have a word with you two.
Beetle: That's why not.
After Maxwell Lord, Batman, and Martian Manhunter learn Blue Beetle and Booster Gold opened a Justice League-themed casino.
Batman: The real question is, where did they get the money for it? Last I heard, those two were flat broke.
Max: Oh, no! NO!*Runs out of the room.* They wouldn't! They couldn't!
Batman & Manhunter: They did.
Max: THEY DID!
Manhunter: If this keeps up he's going to give himself an ulcer.
Batman: He already has an ulcer. He's headed for a nervous breakdown.
*The next panel shows Maxwell sitting in front of a giant computer with his head in his hands. On the monitor is displayed "Available League Funds: $0.00"*
While Animal Man's treatment as a completely useless hero was rather harsh and unfair, his dialogue while trying to find a nearby animal to "absorb" it's "powers" as a fight broke out always cracks me up:
Animal Man: Hold on, guys, while I... *Sees nothing but a small mouse nearby* While I...
Even better — he doesn't wake up from that punch for two issues.
Black Canary nearly goes into a Heroic BSOD over having missed it.
Canary: Oh God, I'm depressed...
One time Guy attempted to use his ring to quickly salvage a wrecked spaceship. The way he did it caused the ship to snap in half. A visibly furious Guy has a large speech bubble with only a "*" in it. The footnote on the panel reads "Expletives. Lots of 'em."
The time Mr Miracle tried to land the League ship on the roof of their new brownstone headquarters . . . and ended up destroying the top two floors because earth buildings aren't always designed to support large aircraft.
Anything with the Injustice League, who in this incarnation were an entire assembly of Ineffectual Sympathetic Villains. Taking the cake, though, would be when they turned over a new leaf and joined the League, and got assigned to Antarctica. And even there they got into trouble, fighting against a horde of killer piranha penguins.