So there's this rumor going around that in the Japanese version of this episode, Takeshi says that right after the Butterfree mate, they die. I guess the idea is that some real-world butterflies die after they mate and that Butterfree, in the Japanese version, is only following suit. There are other versions of the rumor state that the Pokemon Bestiary is the one that actually gives this fact despite the fact that the device doesn't have a single line of dialogue in the entire episode.
Either way, the idea is that Butterfree dies and 4Kids edited this out to prevent the kiddies from being upset at the idea of one of their pokemon friends biting the dust.
This rumor isn't true in the least. Butterfree is still very much alive.
Finding out how a rumor this old got started is a little bit on the impossible side, especially since the rumors are pretty vague when it comes to exactly when in the episode this explanation is supposedly given. However, I can say with a fair amount of certainty that this is the line that the rumors refer to:
My English Translation
English Dub Dialogue
Takeshi: "Umi o watarasanai to, shison nokosu naku naru"
Takeshi: "It if doesn't cross the ocean [and mate], it won't leave behind any offspring."
Brock: "If you don't let it cross the sea, it will never have babies."
Takeshi also says pretty much the same thing at the end of the episode when everyone's bidding Butterfree adieu.
If I had to guess, I'd say that the reason for this "Butterfree dies" rumor is that someone mistook the word shison (子孫, "offspring") for the word shisou (死傷, "injuries and death"). It's an easy enough mistake to make, I suppose, provided you completely ignore the rest of the sentence and zone in solely on the word "death." For years, though, people just took these fans' word for it, convinced that 4Kids was trying to shield us from the horrible, horrible truth.