Continuity mistake: In this episode, Lucy goes out on the window ledge. In "Lucy Cries Wolf", the window ledge was very different: the walls were made of dark brick instead of concrete blocks, there was no downspout, and the ledge went into the corner and stopped, no continuation on the other wall.
Continuity mistake: In the scene where Superman comes to Little Ricky's party Little Ricky has on a birthday hat. When Superman picks him up the hat is gone; it reappears when Superman gets ready to leave.
Revealing mistake: While Lucy is out on the ledge, if you look at the pipe as it runs from above the ledge to below it, the two pieces of pipe don't line up. They are off about the full diameter of the pipe. Also when the pipe pulls out, it's obvious the pipe is on a hinge at the bottom and not really attached to the lower pipe. Later, there is also no stream of water falling from above where the pipe was supposed to be connected as a drain even though it is pouring out.
Answer: There is no reason. It's a just a silly plot device, typical of the era. Women characters were often portrayed as making uninformed assumptions or decisions.
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