I'll add my 5cents :) For "tomber à l'eau", Caroline said it good, it means that what you expect will not happen (I planned to eat with Tom at noon but he said he couldn't be available > Mes plans tombent à l'eau) For "virer à l'échec", it's more about that: some agreements/negotiations which fails - depending to the tense Cela vire à l'échec > it's failing Cela pourrait virer à l'échec > It could fail Cela virera à l'échec > It will fail Hope it helps

If your plans "tombent à l'eau" it means it's not going to happen (fall through). I'm not sure about "virer à l'échec". It sounds unusual to me.