I’ve noticed that milk leaves ugly stains in leather clothes, and those stains CANNOT be washed out! Why? It’s the cow’s revenge or something?
Any kind of liquid is bad for leather. Should I ask what you were doing? 
Seriously, talk to your friendly neighborhood dry cleaners and see what they say. Generally speaking, you’re not supposed to wash leather. Some kinds of leather (tennis shoes, for example) have been processed so as to make them sort-of machine washable, but then you probably don’t care what a pair of $15 sneakers looks like when it comes out of the washing machine. A $500 leather jacket is something else, though.