What do you believe leads to the largest waste of money in the product development process?

Well , Its a nice question.As per me if the product is not running in market at least more than 5 years, and it shouldn't break even with in one year the the product development process is completely waste of money.

If it is in between product development stage we can understand from the prototype alpha, beta stage.

Not following a protocol, the product development starts with a management process, idea generation, investigation, business case,  development, test and validate,launch and post launch review. Business case should be the stage were you check the feasibility of a project. if these steps are not followed, you endup with a non tangible good that is not sale-able. all the stake holders when in agreement will contribute to the success of a project/product. Knowing the regulatory standards, available resources, pilot plant tests can save a lot of money, without which haphazardly launched products will be a waste of time, energy, resources and ultimately money! 

For technology products I believe the lack of early market feedback, or any features beyond minimal viable product have the biggest potential to cause waste. Given the rate of change, speed to market if often the biggest constraint.

Not fallowing the standards and process , documentation and work instruction which leads to rejection and waste of reasources