Normally I'd agree with this, but in the context of the OP's worries, I'd say this is the exact sentiment (perhaps taken too literally) that is paralyzing them

Normally I'd agree with this, but in the context of the OP's worries, I'd say this is the exact sentiment (perhaps taken too literally) that is paralyzing them with fear, uncertainty, and doubt. perhaps a better takeaway is to learn to deliver what you promise (and learn to promise what you know you can learn to deliver). Either way, you can often land the project (at a slightly lower rate) by being upfront with the client about the fact that it will be a learning process for you. Which comes full circle to properly managing client expectations.