In an attempt to save her friend, Dorothy grabs a nearby bucket of water and throws it on him. Unfortunately for the witch, she was standing right behind Scarecrow as he was splashed, taking on some water herself. In a twist that would be stolen and subsequently ruined by M. Night Shyamalan, the witch melts in her clothes -- much to everyone's astonishment.
Why it's baffling:
No one foresaw the witch's weakness being water. Except, you know, perhaps the witch herself. She had to know, right? Having lived with that weakness her whole life?
So why exactly does she have random buckets of it lying around her castle?
A little clean-up saves a lot of trouble.
It'd be like you keeping big, open buckets of hydrochloric acid sitting around your living room. Yes, there are uses for a bucket of water around a castle, true, but how clean do one's floors need to be when one is a) a witch, and b) deathly allergic to water?
And even if she absolutely had to have water in the building somewhere for whatever reason, why did she decide to fight Dorothy and company in the presence of it -- especially when her plan involves setting fire to a walking sack of straw?
The woman has the mentality of a video game level boss, leaving a bunch of ammo and power-ups lying around her fortress, right in the path of the hero. Is it all just an elaborate way to commit suicide?
"Sweet, sweet death."