In this episode, it is made clear that Leonard snores, and has been doing so for years. In The 43 Peculiarity (2012), Sheldon cites Leonard's non-snoring as one of his strong points. Also, in The Einstein Approximation (2010), Penny tells Sheldon that Leonard doesn't snore.
When Sheldon is trying to explain to Leonard how likely he is to die as a result of the surgery, he lists the probabilities of various events (such as asteroid strike or dying on the way to the hospital) on his white board. Beneath them, he calculates the probability of Leonard dying as a result of any one of these simply by adding these probabilities together. This is mathematically incorrect. The correct combined chance is calculated by listing the probabilities that each individual disaster does not occur, then multiply all these values and subtract the result from one.
In Sheldon's board showing the probability of Leonard's death, he listed "8000" (pneumothorax) twice, and left out "1500" (pulmonary edema).