http://www.manic-expression.com/apps/blog/show/34332692-top-5-dumbest-moments-from-the-big-bang-theory-

#5. Leonard contrively advances the plot ("The Work Song Nanocluster")

#4. Amy and Penny run and scream like little girls ("The Infestation Hypothesis")

#3. Is Sheldon Autistic? (from "The Luminous Fish Effect")

#2. Sheldon and Leonard Fail Basic Trivia ("The Bat Jar Conjecture")

#1. Evil Wil Wheaton and the Raiders of the Lost Ark ("The 21-Second Excitation")

I certainly agree about #3. That episode was very out of character for Sheldon. #2 is another good point: Leonard and Sheldon act like they have never heard of the questions Penny is asking. It just doesn't make sense.