The Most Common Mistake?

Iterative and agile process coaches often see scenarios like this:

Jill: Sure, we don't apply the waterfall everyone knows it doesn't work. We've adopted <iterative method X> and are into our first project. We've been at it for two months and have the use case analysis nearly finished, and the plan and schedule of what we'll be doing in each iteration. After review and approval of the final requirements set and iteration schedule, we'll start programming.

This profound misunderstanding, still superimposing waterfall-inspired, big up-front analysis and planning (predictable manufacturing) values onto iterative methods, is one of the most common mistakes that new iterative and agile method adopters make.