A failed fusion can only occur when fusing using the Fusion Dance in an incorrect way. Many of the fusions in the series have been performed incorrectly, leading to a failed fusion. There are examples of failed fusions from Dragon Ball Z, from movies, and from video games.
Veku (ベクウ, Bekū) is the obese version of Gogeta who appears when Goku and Vegeta attempt to fuse for the first time in the movie Dragon Ball Z: Fusion Reborn against the demon Janemba. King Kai blames Vegeta for the mistake, claiming that Vegeta has failed to extend his index finger, resulting in an embarrassingly weak fusion which forms a fat fighter called "Veku" by South Kai; a name that comes from Vegeta and Goku. In this form, speed is decreased, and ki is extremely limited.
Veku is unable to perform any special ki attacks, due to his severe ki limitation. Also, because of his fat body, he cannot retaliate by punching and kicking. Instead, all he can do is pass gas, which he uses against Super Janemba, and escape using what he calls his "Rabbit Feet" technique. Like most fusions, there is a 30-minute time limit before the two fighters defuse. Veku is voiced by both Sean Schemmel (Goku) and Christopher R. Sabat (Vegeta), using the voices of King Kai and Yajirobe, respectively.
Failed Versions of GotenksEdit
Goten and Trunks achieve this form when the Fusion Dance goes wrong, probably because Trunks stretches his finger at an inappropriate time. The result is a fat version of Gotenks. He tries to run but tires very quickly. For a moment, Krillin and Yamcha think that the fusion has succeeded, and that Gotenks has to be fat to regulate his power, similarly to Majin Buu. However, they soon change their minds when they see that fat Gotenks becomes winded after a brief and slow run. Fat Gotenks is also seen in Dragon Ball: Yo! Son Goku and His Friends Return!!, Goten and Trunks as Fat Gotenks say that they did not practice the fusion dance for a long time, and then they defuse. He is a playable character in Dragon Ball Heroes, but in other video games, he can not do any special attacks, and is already tired after a few kicks and punches. In the case of the 2004 RPG Dragon Ball Z: Buu's Fury, neither he nor the skinny variant can even move, and defuse automatically after a few seconds so that a successful fusion can be performed.
Goten and Trunks achieve this form of Gotenks when the Fusion Dance goes wrong for a second time. The failure is due to a lack of coordination, as when Goten and Trunks say "HAAH," their fingers do not meet. Skinny Gotenks can barely stand on his feet without getting tired, and he coughs a lot as well. Comically, after seeing this form of Gotenks, Chi-Chi exclaims "I can't love that!" to which Master Roshi asks "Why not? He's my age!", Krillin responds with "No...older!"
This form of Tiencha arrives in Dragon Ball Z: Budokai 2, when Yamcha and Tien Shinhan stretch their hands when they say "HAAH" in the last stage of the Fusion Dance, a costly mistake. In this form, Tiencha can still fly but can't do any special attacks. He can only use punches, kicks and simple ki attacks, and even these cost him a large portion of his energy. After a combo attack, he coughs, and if he tries a special attack like the Dodohameha, he again coughs, causing the attack to fail.