In my first playthrough of Ruby, I found a wild Manectric and thought it was a new Legendary Dog.
In my first Silver playthrough, I had two Ledyba in the daycare. I didn't know Pokemon could make eggs at the time. Every time I talked to the Daycare Man, he had an egg for me. I thought I was super lucky, and kept collecting all the eggs. I hatched every one of them, only to find out they were all Ledyba. I didn't know why he kept giving me the same Pokemon egg. *facepalm*