Any presentation of a Lifetime Achievement Award can count, but especially Charlie Chaplin's.
From 2008: Jon Stewart bringing Markéta Irglová back onstage so she could give her acceptance speech for "Falling Slowly" as Best Song, after she was played off before she could say anything.
That was actually the late Oscar producer Gil Cates's idea to bring her back. He found out that Lou Horvitz, the director, had mistakenly given the cue to the orchestra, so during the commercial break, he asked Stewart to let her come back on stage and finish her speech.
From 2005: Jamie Foxx's speech after winning Best Actor for Ray, especially when he choked up speaking of his deceased grandmother, who he said was his first acting teacher. Check it out for yourself.
From 1994: One of the cutest moments in Oscar history, Anna Paquin's reaction to winning Best Supporting Actress for The Piano (skip to 0:54 or 0:55 and look at her face when she realizes that she's won). She was only 11 years old at the time and you can tell she really did NOT expect to win. After being handed her statuette, Anna just stood at the podium hyperventilating and giggling for about 20 seconds, like the little kid she was at the time. Even the audience in their seats couldn't help but laugh at how adorable she is.
Extra points for how genuinely happy Holly Hunter and Emma Thompson were to see Anna win. To clarify, Hunter and Thompson were also nominated in that category, and the former would win Best Actress for the same film later that evening.
From 1990: Daniel Day-Lewis's first Best Actor acceptance speech for My Left Foot, where he made sure to give special attention to Hugh O'Conor, who had played Christy Brown as a young boy and was just as convincing.