I'm guessing it was A Bug's Life (1998), but were there any previous animated movies to put mock bloopers in their end credits?
According to Pixar Wiki, A Bug's Life (1998) was indeed the first Pixar movie to have outtakes.
I've not found any non-Pixar films with bloopers before this, and the outtakes and bloopers in A Bug's Life were certainly noted at the time for being unusual; a few weeks after release, a second set of bloopers were added to encourage return viewings.
Here are the first three bloopers listed for A Bug's Life:
- Hopper is doing his line of "Are you saying I'm stupid? Do I look stupid to you?", but Atta can't help but burst out laughing, so they try the take again and she's still laughing. The third time, Atta says, "Yes!" with more laughing. Hopper is upset and says: "This is the fifteenth take, I cannot work like this, I will be in my trailer." Atta then says, "I need a break."
- Rosie is doing her line of telling Flik that they are not fighting Grasshoppers, but is interrupted by the sound of someone hammering off-screen, so she tells them to stop. The bell rings and Slim asks if they can see him in the shot.
- Flik grabs a piece of dandelion and shouts out Buzz's catchphrase of "To Infinity and Beyond!". The crew and Flik starts laughing. Flik apologizes, saying that he couldn't resist. He carefully places the dandelion piece back and says, "Let's go for real," and apologizes again.
Toy Story 2 (1999) was second to have outtakes, and Monsters, Inc. (2001) "was the third and (so far) last Pixar movie to have outtakes".
However, "although [The Incredibles (2004)] did not have outtakes at the end of the movie, the 2-disc collector's edition DVD featured a segment called "Incredi-Blunders", which were accidental animation mistakes from the making of the film."