David Freedman gives two examples of animal-to-human studies that went horribly wrong. One actually happened. The other is hypothetical.
The actual study involves the experimental drug TGN1412. The compound was found safe in animal studies at 500 times the dose that would be given to humans. In 2006, TGN1412 was administered to six healthy men. All six were in excruciating pain within an hour of receiving the drug. Within 48 hours, all six were experiencing multiple organ failure. One subject remained in intensive care for several months. More information is available in this report.
Safety in animal studies is necessary but insufficient for testing new compounds in human subjects.That is, compounds that are harmful to animals do not go on to testing in human subjects. This policy is eminently reasonable. However, some drugs that would have been safe and effective in humans are discarded because they were toxic in animals. From Freedman:
It is frequently claimed that penicillin might easily have become one of those mistakenly discarded drugs because it sickens rabbits and guinea pigs in large or in oral doses.
In other words, animal testing might have blocked the development of one of the most important drugs in the history of medicine.