Continuity mistake: In the laundry room when Penny is talking to Sheldon, Sheldon takes some time arranging a blue shirt on his plastic laundry folder. Then it suddenly turns into a pair of socks, which he folds and then picks up the blue shirt again. There was not enough time for him to switch out the shirt for the socks. (00:06:30)
Continuity mistake: When Sheldon is sitting with Raj in his apartment watching television, the can of soda on the table in front of him rotates itself a couple of times. (00:14:40)
Continuity mistake: Right at the start of the episode, Leonard kisses Penny. When he starts kissing her his head is tilted to the right (opposite side to the wall with the camera). When they move to the camera image, his head is tilted to the left (on the side with the camera), and when they move back his head is back tilting to the right again. (00:00:30)
Continuity mistake: Leonard says that Sheldon has gone crazy and chucks his pen down on a red pad on the desk. When the shot changes, so does the position of the pen without being touched. (00:13:10)
Continuity mistake: When Sheldon is undoing a pair of socks that Penny has paired, Penny is seen holding a single sock. Camera changes and she now has nothing in her hands. This is an instant cut. No time to drop it. (00:05:50)
Continuity mistake: When Sheldon is looking at his socks that Penny paired up, his grip on them changes between shots. (00:06:00)
Continuity mistake: Penny throws Sheldon's sock down on the table, and their positions change when Sheldon picks them up, despite no one touching them. (00:05:50)
Continuity mistake: When Leonard shows Penny the leaflet at the end, she is holding it in one hand in shot but with 2 as the camera changes. (00:19:50)
Continuity mistake: In the laundry room, Sheldon is holding a blue shirt in his hands and has a bit of extra material near his hands. When the shot changes, he is holding the shirt differently and the extra material has gone. (00:05:50)
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