Usopp’s lies all have a way of becoming true and this is no coincidence. His lies have always been his dreams and ambitions and as he travels more, Usopp is realizing these things. Some examples are:
The Giant Goldfish:
Usopp told Kaya about a giant goldfish that he saw in the southern sea. The goldfish was so big, that its poop looked like land. After their adventure on Little Garden, Luffy and crew encounter the Island Eater, a gigantic goldfish that indeed looks big enough to eat an island. Dorry and Brogy recall that its feces are so large that they look like an island. They themselves once set foot on Nothing Island, which was nothing more than the Island Eater’s excrement.
The Legendary Mole:
Usopp uses a lie that he was tracking a legendary mole as an excuse for trespassing on Kaya’s property. Usopp ends up battling against Miss. Merry Christmas with Chopper in the final showdown against the Baroque Works. As fate would have it, her ability from eating the Mogu Mogu Fruit is to turn into a giant mole.
While playing with a dog back in Syrup Village, Usopp pretends that it's actually the three-headed Cereberus. This lie turned out to come true...TWICE! The first was the seemingly three-headed Judge from Enies Lobby, Baskerville (as in The Hounds of). The second and most direct appearance of a Cerberus-like character was...well...Cerberus! Usopp actually encounters a three-headed dog on Thriller Bark but technically, it's a two-headed dog. The third head is actually a fox's.
Which lie from Usopp's past will come true next? The few scenes Oda included of him telling lies in Syrup Village are quickly being used up and I seem to recall him mentioning something about a dragon...
In a shocking turn of events during the Water Seven arc, Usopp commits mutiny and challenges Luffy to a duel. These events transpired over Summer 2004 and were a drastic departure from the series' generally upbeat tempo. It was a heart-breaking battle that both took seriously which came at a time whe it seemed the crew was falling apart at the seams. The origin of the grievance came from Usopp wanting to protect The Going Merry which Luffy was prepared to replace as it had taken irreversible damage to its keel. Usopp's stubborness stemmed from his witnessing what he believed was the spirit of Merry. He didn't want to abandon the ship since he saw it as a living entity. Furthermore, when Usopp first 'joined', Oda was very careful in how he worded Usopp's entry into the crew. Luffy technically invited Usopp to be a crew member but Usopp never actually verbally agreed to the invitation. And so, when tensions between Usopp and Luffy were high enough to bend steel, Usopp stated that if he didn't agree with Luffy anymore he could leave just as easily as he 'joined'. Ultimately Oda decided to take the stance that while both sides had their valid points, Usopp was wrong in his actions. Not only for abandoning the crew over the ship, but for rebelling against his Captain. Usopp repaired the bonds he broke by genuinely apologizing for his actions. Interesting to note, even though another crew member left the crew at one point for his or her own reasons, Usopp was the first crew member who was actually considered to have fully left the crew. This is according to the opening pages on the comic compilations which listed him separate from the crew in the Introduction.