Bob plays a Fireball on Sue, for enough damage to kill her. Bob doesn't intend to play any interrupts, but Sue wants to counter the Fireball, so she begins a batch of interrupts while it's being cast. She plays Hydroblast, which counters a target spell if it's red (as the Fireball is). Bob chooses not to interrupt Sue's Hydroblast (as he might if he wanted to counter it) but instead responds to it by interrupting the Fireball and targeting it with Deathlace, which will change the Fireball to black. Neither player wants to play any more interrupts, so the batch resolves. Deathlace turns the Fireball black, and then Hydroblast fizzles, as the Fireball is no longer red.