Anne smashing the slate over Gilbert's head in Disproportionate Retribution for a comment about her hair. The line in the book clarifies that it was the slate that cracked, and not Gilbert's head.
Anne tries to dye her hair Raven black. It ends up green.
Anne and her friends acting out "The Lady of Shallot." It all goes well until the boat sinks...
Anne accidentally getting Diana drunk. Though the consequences aren't funny, the event itself is. What she thought was raspberry cordial turned out to be currant wine, and Diana drank three tumblerfuls of it.
Anne makes a cake for a guest, but accidentally uses muscle linement instead of vanilla extract. Anne had a cold and couldn't smell the difference.
Anne of Avonlea
Mr. Harrison, Anne's irascible new neighbour, tells her to stop letting her cow get into his garden. Shortly afterwards, Anne sees a cow that looks just like hers in his field, thinks her cow has escaped again, and goes to chase it. By the time she catches the cow, she's so thoroughly out of sorts that she promptly takes the opportunity to sell it to Mr. Shearer on the spot. Only afterward does she discover that her cow is right where it was supposed to be - the cow she sold was actually Mr. Harrison's own cow. Fortunately, Harrison takes this all with reasonably good grace when Anne goes to explain and apologize.
Anne of the Island
The girls of Patty's Place trying to humanely chloroform a stray cat, by sealing it in a wooden box... which they totally forget to check for knotholes.
"Why in the world don't you come?" demanded Stella, appearing in the doorway. "We've got the grave all ready. What, 'silent still and silent all?' she quoted teasingly.
"'Oh no, the voices of the dead/Sound like the distant torrent's fall,'" promptly counter-quoted Anne, pointing solemnly to the box.