The Breast Feeding Crisis was a section in Dr. Beverly Hofstadter's book "The Disappointing Child".
The Book
Penny bought the book for her psychology class written by Leonard's mother in the episode The Raiders Minimization. Leonard was very disappointed that she had bought it.
The Breast Feeding Crisis
Leonard explaining the "crisis"
Leonard denied that it was a crisis. Leonard has always went for the left Breast when Breast Feeding. This action resulted in Beverly being lopsided.
Trivia
When having sex with Penny, he still usually goes left.
Beverly also wrote a book about Leonard called "Needy Baby, Greedy Baby"
Raj suggests that if they ever make the movie of her books, Sandra Bullock should play Leonard's mother.
It upset Leonard that she was reading it, so he was able to guilt trip Penny into getting what he wanted.