After taking the Titan Serum, Rod Reiss (Attack on Titan) became a Titan, but as he ingested it orally rather than injecting it, he became a malformed beast too large to even move properly.
Papillon (Buso Renkin) was forced to use an incomplete embryo to transform into a humanoid homunculus to counter Jiro's attempted sabotage, thus he did not acquire the emblem nor could he remove the pangs of his illness.
Mafuga (Doraemon) is a wind demon dragon formed from three wind spirits: Fuuko, Gorado, and a the Red Egg. Even after Fuuko was freed, Gorado and the Red Egg recombined to form a less powerful though more evil Mafuga, as Fuuko's influence was absent.
Cell (Dragon Ball) in his incomplete form, in which his powers pales in comparison to his complete form, which requires the absorption of Artificial Humans #17 and #18 to achieve.
Son Goku (Dragon Ball) was so enraged at Lord Slug that he took on a Pseudo Super Saiyan state against him. While not a true Super Saiyan form, he gained enough power to easily overpower Slug, when he was no match for him before.
During the Tournament of Power, Son Goku (Dragon Ball) achieved an incomplete state of Ultra Instinct. While powerful, and possessing great defensive capabilities and reflexes, Goku's attacks were comparably weaker, since those required conscious effort on his part.
When Vanitas (Kingdom Hearts: Birth by Sleep) attempted to create the χ-blade by merging with Ventus, their fusion was incomplete, leaving the χ-blade in a broken and incomplete, though still powerful, state.
Calamity Ganon (The Legend of Zelda: Breath of the Wild) attempted to reincarnate in a physical form, but was interrupted by Link's arrival, forcing him to resurrect in an incomplete state.
When Ultron Sigma (Marvel vs. Capcom: Infinite) attempted to use the Soul Stone without actually possessing a soul, it rejected him and transformed him into the unstable Ultron Omega.
When a Gore Magala (Monster Hunter 4 Ultimate) is interrupted in the process of molting into a Shagaru Magala, it becomes a Chaotic Gore Magala, an improperly molted creature.
Gaara (Naruto) was forced into an incomplete transformation due to being injured by Sasuke's Chidori. As a result, he could not focus on taking the form of the Shukaku, and was severely hampered.
Mizuki (Naruto) used an incomplete experimental potion from Orochimaru to achieve an animal-hybrid transformation, which gave him an explosive burst of powers. Because of incomplete formula, he suffered cellular destruction backlash that would have killed him had Tsunade not saved him.
The Ten-Tails (Naruto) was revived in an incomplete form due to Obito losing his patience in capturing Gyūki and Kurama, thus resulting in a weakened form.
Becoming the jinchūriki to an incomplete Ten-Tails, Obito Uchiha's (Naruto) form is also incomplete. He did not possess the full powers of the Rinnegan, thus Black Zetsu did not want to use him as the vessel for Kaguya.
Bob (ReBoot) merged with his keytool, Glitch, when it was damaged, thus resulting in a form that can be considered a failure.
HAOS (Resident Evil 6) was awakened before it was fully completed, leaving it in an incomplete, though still deadly and powerful form.
Alf Layla wa-Layla (Sonic the Hedgehog) is the form the Erazor Djinn takes after absorbing the World Rings and killing Shahra the Ring Genie; however, as Sonic who collected the World Rings, was not sacrificed, Erazor did not gain full control over the rings, resulting in his transformation being incomplete.
Dark Gaia (Sonic the Hedgehog) was awakened in an incomplete state when it was prematurely awakened by Dr. Eggman.
Using four Chaos Emeralds, Turbo Mecha Sonic (Super Mario Bros. Z) enters a "semi-super" state. While not his true super form, he was powerful enough to trounce the Axem Rangers X and Mario's team all at once, and generate an energy ball powerful enough to annihilate Yoshi's Island.
After suffering a brutal beating by Splinter, the Shredder (Teenage Mutant Ninja Turtles 2012) underwent a mutagen treatment to become the Super Shredder. While incredibly powerful, the mutagen formula he used was unstable, causing side-effects such as his arm to painfully shrivel up.
By cannibalizing other Ghouls, Ken Kaneki (Tokyo Ghoul) mutated into a Kakuja, developing a secondary kagune that serves as natural armor and enhances his strength. Because the transformation is currently incomplete, he lost his sanity and become a psychotic berserker.