The Luminous Fish Effect - S1-E4
Continuity mistake: When Penny comes into the apartment to ask if Sheldon needs anything from the market, between the few shots, the position of the napkin and the pile of eggs on the plate he is holding changes. (00:05:15)
Hamster
The Luminous Fish Effect - S1-E4
Continuity mistake: When Leonard and Sheldon are walking downstairs, Sheldon stops and pins back up the caution tape that is in front of the elevator. Between shots, the other strip of caution tape untwists itself. (00:01:25)
Hamster 
The Luminous Fish Effect - S1-E4
Continuity mistake: When Sheldon's mom is saying the prayer, the bread roll that Raj is holding swaps hands, and then it disappears in the shot where she tells them they don't have to join in with the end of the prayer.
Hamster 
The Luminous Fish Effect - S1-E4
Continuity mistake: When Penny is asking if Sheldon has ever done anything like this before she is holding her plate with 2 hands in one shot and 1 hand in the next. (00:13:20)
Ssiscool The Luminous Fish Effect - S1-E4
Continuity mistake: When Sheldon asks his mum why she's there, his mums hand on his chest changes position. Note the distance to his elbow. (00:11:40)
Ssiscool The Luminous Fish Effect - S1-E4
Continuity mistake: At the buffet, Sheldon has 2 hands on his plate. Camera cuts and now he has one hand by his side. This is before Howard walks in with his date. And the cut is instant. (00:02:00)
Ssiscool 
Chosen answer: The song is called "Dark as a Dungeon" and was written and first performed by singer-songwriter Merle Travis in 1946. It has been performed by a wide array of artists, including Tennessee Ernie Ford, Harry Belafonte, Dolly Parton, Queens of the Stone Age, Kathy Mattea and Amy Grant. But it was made most famous when it was performed and recorded by Johnny Cash during his concert at Folsom Prison in 1968. According to Wikipedia: "It is a lament about the danger and drudgery of being a coal miner in an Appalachian shaft mine. It has become a rallying song among miners seeking improved working conditions."
Michael Albert