It's not over until the fat lady sings

It's not over until the fat lady sings
 

I was listening to music with a Peruvian friend recently, and there was this English idiom. I had difficulty explaining it to her. Is there a Spanish idiom that means something similar? :)

Nada es seguro hasta que suena/e la campana

or

Nada está/estará dicho hasta que suena/e la campana.

otros:
"el partido no se acaba hasta que pita el árbitro"
"la obra se acaba cuando baja el telón"

Saludos :D