There isn’t much you can do – not as a consultant. You can re-phrase your position. You can argue until you are blue in the face. But you can’t win.
What you can do though is start planning for recovery. If a client is making a mistake your next job is to minimize the fallout from that mistake. You need a backup plan. And depending on the client, you might have to devise a plan that they think they came up with. Who cares? Your job is to help, not to take credit.
The key though is always thinking through contingencies – the “what if’s”. You need to think about what your client isn’t thinking about – otherwise they don’t need you. You need to be able to look beyond the things that are blinding them and show them alternatives – present them choices, give them room to maneuver within the overall goals of the company.
You add value when you offer a different perspective. Your unique perspective is what you have to offer – it is the valued service you are getting paid for. Don’t be afraid to disagree with your customer – but never forget who you work for. Disagree respectfully, offer alternatives, and make it clear the decision is theirs to make. But always be ready to offer a “way out” of bad decisions. That’s really what you are getting paid for.