St. Jerome called the leaven heretical doctrine (so fashionable at Seminaries). John Calvin insisted it was atheism (so fashionable at Universities). And though both Jerome and Calvin may have been right, in a comment below I claimed that Christianity should (but too often doesn't) do the job of deconstructing itself, so I thought I'd give an example:
In his commentary on Matthew, in order to avoid such scapegoating Dale Bruner consistently renders the traditional "Pharisees and Sadducees" as "The Serious and the Sophisticated." It's the best single recent English Bible translation maneuver I'm aware of. It delivers the medicine to those who need it most: The self-styled "pious" and "intelligent" who would actually bother reading his commentary.
Then Bruner goes on to uncovers what this leaven actually is:
"All three Synoptic evangelist understood the application of Jesus' leaven warning differently: Mark understood Jesus' warning politically ('Watch out for the leaven of the Pharisees and Herod,' Mark 8:15); Luke understood Jesus ethically ('keep yourselves free from the leaven of the Pharisess, which is hypocrisy,' Luke 12:1); and Matthew understands him doctrinally. 'watch out for their doctrine.' Matthew is the theological Gospel. Matthew hears Jesus warning us specifically about the perfectionist theology of the Pharisess and the secularist theology of the Sadducees. Jesus thus calls all later disciples to discern in their own time the perfectionist (superconservative) and secularist (supermodern) theologies of their age and to free themselves as much as possible from both of them. This leaves disciples with the teaching of Jesus - alone" (p. 116).And needless to say, before anywhere else the place to look first for the superconservative/supermodern leaven of pride is within.
One sign might be a blogger who consistently quotes himself in his own posts.