Ice Cream Sand Witch

But it failed!

I just so happened to be looking up the specific mechanics of Transform earlier today, funny how that works out. Basically, like Arkhi said, it will fail if used on Ditto or a Pokemon already transformed- with the notable exception of Gen I. In Gen I, if you are in a trainer battle where both you and your opponent only have a Ditto left, the battle will never end because Ditto continually transforms into itself, restoring its PP every time it does so. Since you can't run from trainer battles and Ditto can only learn Transform, you will be forced to soft reset your way out of the battle unless you have an infinity to kill.