The "On the set of D&D 4th Edition" cartoons had quite a few gems, not the least of which was the infamous, "I'm a Monster, Raar!"
The first edition Monster Manual includes a picture of a man with his upper half inside a giant frog. The man is bend 90 degrees at the waist and not struggling, so the result reminds one of a lion tamer putting his head in the lion's mouth.
In Fiendish Codex: Tyrants of the Nine Hells, there's a section detailing Deals With The Devil, as well as how to get out of them. It's possible to do so legally, in Hell's actual court, but only if the bargaining devil actually broke the law (such as by actually lying to the mortal). Then it ends with this gem:
It is also possible for a defendant to win her case on merit, only to suffer condemnation to the Nine Hells on unrelated grounds. Much diabolical laughter ensues.
The third edition Dungeon Master's Guide has two gems both related to naming: first, when speaking of whether to make the game style serious or humorous, it notes, "If the king of the land is a talking dog named Muffy or if the PCs have to find a brassiere of elemental summoning rather than a brazier of elemental summoning, don't expect anyone to take the game too seriously." Then under naming conventions it suggests, "Although any character name is fine in and of itself, a group that includes Bob the Fighter, Aldrorius Killravenof Thistledown, and Runtboy as characters lacks the consistency to be credible."
The fourth edition mentions the following "In a group consisting of Sithis, Travok, Anastrianna and Kairon, the human fighter named Bob II sticks out. Especially when he's identical to Bob I, who was killed by kobolds ... Travok and Kairon don't want to visit Gumdrop Island or talk to the enchanter Tim." This excerpt is a textbook example of Aerith and Bob.
The comic book by John Rogers opens with the line, "On the bright side, they're orphan zombies, so nobody's gonna miss 'em," and only goes on from there.
The Forgotten Realms sourcebook Races of Faerūn contains this gem: "Human legend has it that the centaurs are the result of some mad cross between a wild elf and a wild mustang, but both the wild elves and the centaurs take umbrage at this suggestion. (The mustangs have no particular opinions on the matter.)"
In "Search for the Dungeon Master" Presto demands of his hat something to stop a charging monster. His hat produces a Stop Sign. Presto's sheepish grin as he plants the sign and runs away sells it.
Quite a few from the first half of "The Girl Who Dreamed Tomorrow."
First, Eric, in a panic over the Bullywogs, tries to warn his friends, while they ignore him, resulting in him grabbing and shaking a Lizard Man to try to get his point across, only to realize a moment later who he was talking to.
Sheila then volunteers to retrieve him from the Bullywogs and Lizard Men, all of who are fighting over who gets to capture him. Invisible, she slaps a Lizard Man on the ass, and then calmly retrieves Eric, still gibbering, from the ensuring chaos.