"The Circus is in town, Dicurion. Make a donation to the good priests of the Laughing God. See if he won't smile down upon us, and wish our little problem away."
-Olrich van Haarkrom
If a creature is transformed into another creature (say, a human into a dog), then that creature will become more and more accustomed to the new form until a point is reached where reversing the spell is impossible. A second transformation is possible, but the creature will have to go through the whole process again.