Title screen gag
Wikisimpsons - The Simpsons Wiki
When The Simpsons opening sequence was reanimated for its conversion to high definition in 2009, initially, during the screen showing the title, only one gag was used: a three-eyed crow flying by. Starting with the season 21 episode "Bart Gets a "Z"", the gag would become a changing gag for each episode just like the billboard gags, the chalkboard gags, and the couch gags. The gag consists of something moving across the title screen reading "The Simpsons" among the moving clouds.
 Title screen differences before season 21
There have very rarely been differences in title screens before season 21. An example is in "The Simpsons 138th Episode Spectacular" (pictured) where the font is different and "138th Episode Spectacular" is written underneath it.
Another would be the episode "Treehouse of Horror" which has "Halloween Special" added underneath the title.
 See also