This slip-up is in the episode where Lucy thinks Ricky is trying to kill her. Watch the scene where Ricky puts a "mickey" in Lucy's glass. Pay attention to the glasses. After she switches the glasses when Ricky isn't looking, then he switches them back when she isn't looking, she winds up drinking the glass that she left for Ricky.
This is what I see:Ricky really drinks the glass with the sleeping powder in it. when Ricky first hands Lucy the cup with the sleeping powder in it, Lucy puts it on the table on the right. When Ricky dances she switches the glasses so the one with the sleeping powder in it is on the left. Then Lucy says "That's enough dancing lets drink." Then she was about to pick up the glass with the sleeping powder in it on the left when Ricky says "Look at that. That is the biggest spider I have ever seen!" Then he switched the glasses back so that the sleeping powder was in front of him on the right. He drank the one with the sleeping powder not Lucy.
But the slip-up is that she actually doesn't drink the glass with the sleeping powder in it. She drinks Ricky's glass. Ricky's glass, which Lucy knocked out of his hand, had the "mickey".
I think I missed something in here; it sounds more like a GAG then a slip up. Look at every movie made with a mickey scene, they switch around the glasses then they suddenly drink the mickey or they do it right (hardly ever) and then they end up giving the mickey to the other person. If it was a slip, up she would have knocked out the un-tainted glass from Ricky's hand or something like that.
But they showed this "mistake" on the I Love Lucy DVD. In the flubs section, they actually go to the part of the show where the goof happens, and show it to you carefully.
Did you watch the rest of the episode? Of course she drank from the wrong glass. That was what the script directed. Ricky even told her he slipped her a "mickey". Of course, it was only sleeping powder . . .
NO this was not a mess up. If you would've kept on watching. She takes the glass and throws it and says,"No I can't do that to you even though you did it to me." He says "what are you talking about?" or something and then she says "I switched the glasses." and he says, "I know I switched them back" and then she says, "you mean i got the..." he says something like, "you sure did!" then she grabs her throat and starts to gasp. Watch the full half hour next time pal.