Character mistake: After Walt tries to slap the fly on the paper, he lifts his hand and the math is wrong on one of the sums. He has 6,012 + 7,287 = 13,301. But the actual answer is 13,299. (00:05:45)
Continuity mistake: The amount of glass on the floor changes after Walt breaks the light. (00:07:10)
Audio problem: When Walt yells at Jesse through the lab's entrance door, the sound is very muffled, as if the door is a major insulator of sound. However, later, when Jesse yells at Walt through the same door, the sound is much clearer, and hardly muffled at all. (00:22:15 - 00:23:55)
Chosen answer: It is part of the ritual for la Santa Muerte. It is a sign of humility and sacrifice.
djm