Lucy Thinks Ricky is Trying to Murder Her - S1-E1
Corrected entry: Halfway through the episode, Ricky is talking on the phone about props for that night's show at the club. You can hear both sides of the conversation. At the end of the call, Jerry on the other end says, "See you later, Des" to Ricky. He used Desi Arnez's real name instead of his character's.
Lucy Thinks Ricky is Trying to Murder Her - S1-E1
Corrected entry: In the scene where Ricky tries to get Lucy to have the drink that will put her to sleep, from the audience's point of view, Lucy is on the right side behind the living room table with the altered drink on the table in front of her. Ricky is to the left with the unaltered drink in front of him. While Ricky is repeating Lucy's spinning/clapping movement, Lucy switches the glasses around. When they are done dancing, Lucy ends up behind the table to the left, with the altered glass again in front of her. Ricky points to the wall and tells Lucy, "That is the biggest spider I have ever seen!" While Lucy is examining the wall, Ricky switches the glasses back around. The unaltered glass is now in front of Lucy. She then proceeds to drink while Ricky smiles at her plan's apparent "failure". If you have watched the glasses carefully, you will realize that Ricky is actually the one who ends up with the altered drink, not Lucy.
Correction: For years I also thought this was an error, but it's not. When Lucy first starts out with the "poisoned" cup, it's the one on the audience's right. After Lucy switches it once and Ricky switches it back, the cup on house right IS back in the same place, however Lucy wasn't drinking from that cup, but took the cup at house left (the "unpoisoned" cup) So the episode is correct.
Correction: He says 'dad', not 'Des'.