Martin Fowler has a good post on the Fixed Scope Mirage.
Many companies like the idea of writing a contract that fixes scope and price because they think it lowers their risk. The mirage says that their financial obligation is fixed at the price of the deal. If they don't get satisfactory software, then it won't cost them.
I see this often when looking for new projects. The potential client has a project they want done, and they want it done in X weeks/months. Why? Because they're willing to pay X dollars for the software and their next door neighbor (whose in IT) did an estimate for them while they were drinking beers the other day. I've learned my leason with these clients - run! Any client that estimates how long it'll take you to do something is going to be a nightmare to work for. Furthermore, if you get the gig - you've likely told them that you could do it in their timeframe.
Even worse are clients that want you to do a 1-2 week project. These are a nightmare because they tend to want lots of functionality, but don't want to pay for it - hence the 2 week project. From my experience, I've learned to only take clients that allow you to do the estimating and have projects that are a month or longer.