*Nessus' Bane isn't the only perfect potion. Tasteless, odourless, undetectable - there's only been one other quite like it. Suddenly the perhapses of before become certainties, and the plot solidifies around him. The torture before had worn him down, the water over his head had caught him off guard and Occlumency never can work when the threat has already gestated inside you.

The sting of such an amateur mistake hits Snape before the implications.*