A Big Piece of Garbage - S1-E8
Other mistake: When Professor Farnsworth was presenting the crude drawing of the Smelloscope to his scientific peers, he wiped the napkin which it was drawn on on his forehead, causing all the ink to smudge. But why isn't there any ink on Professor Farnsworth's head? (00:05:30)
Answer: Yes Leela knew from early on that Fry had feelings for her. They were pretending to be a couple in 'A Flight to Remember' from season 1 and Fry tried to get on dates with Leela ever since. She just didn't show signs of the feelings maybe being mutual before some of the later episodes.
Andreas[DK]