Some people just have bad reactions to potions. Tom had been looking forward to using the Transformation potion to go out as Vicki, the cute girl who worked in his office. He had the hair and nail clippings needed to make the potion give him her shape. He’d done everything right according to the directions. When he quaffed it, he knew there would be some discomfort as his body shrank down and morphed to look like Vicki, but he wasn't prepared for this.
He wasn't shrinking, he was growing and expanding. His arms became smooth, and his hair lengthened, and breast grew on his chest, but he was shooting up in height. At ten feet, she had to duck her head and bend over. At twenty feet, she had to bend her knees to not bump the ceiling. Finally, she stopped growing, and her new clothes formed around her.
Oh she looked like Vicki alright; a 20’ foot Amazon Vicki. She couldn't go out like this, hell she couldn't fit through the office doors. Her only hope was to hide out and wait eight hours for the potion to wear off. Of course, with a reaction like this, who’s to say how long the effects might last. Tom might be stuck as a giant girl for weeks.