I had one once who came in with a brilliantly produced business plan, forecasts, projections, branding, business cards, etc. She was a business/economics graduate and the business was an e-commerce site which she was showing would grow massively into a multi million pound enterprise within 3 years at which time she'd sell it for £50 million. I must admit, I was impressed as she'd "ticked all the boxes" for professionalism and must have spent hundreds of hours on drafting it all, researching, etc. Usually, when new clients come in, they're "light" on the numbers which is where we specialise (i.e. forecasts, tax implications, KPIs etc) but she'd done all that.
The only "pages" which were blank were about the product she was selling - the pages on suppliers, customers, products, selling prices, margins etc were all "TBA". At first, I didn't think too much about that as many new clients "anonymise" their business idea in case it's stolen so it's not something we hadn't seen before. She even insisted on a non disclosure agreement with us, which is also pretty unusual as our usual terms of business already include confidentiality which is fundamental to our profession so a NDA isn't really needed, but we sign them anyway to give peace of mind.
Fell at the first hurdle when I asked her what the product was. She basically hadn't a clue! She'd managed to do a full business plan/modelling exercise without knowing what she was going to sell, where she was going to buy it, who her customers were, buying/selling prices, competitors, etc. I was absolutely dumbstruck. She actually thought that sourcing products to sell was the easy bit and that when she started looking for suppliers, it would all fall into place - she'd just find a few wholesalers and take it from there. She completely failed to understand that the market is saturated with people buying from bog-standard wholesalers.