By mistakes, do you mean as in the failure to maximize profits for shareholders?
Overall, I think Disney suffers what ails many large companies in that they are slow to change and rely too much on cost-cutting instead of boosting revenue. It's like a giant ship that turns ever-so-slowly.
1. Slow to target more affluent guests who demand better service and higher-quality experiences. Demographic trends have favored the wealthy and Disney was built around attracting the middle class, which has been shrinking away.
2. Overbuilding in Florida. Was that fourth park really necessary while EPCOT fell stagnant?
3. Slow to update attractions and experiences.
4. Reducing the overall experience by cutting costs
What may be a "mistake" in maximizing profits is not the same as an experience you may want or miss.
For example, I'd love five parks to visit, double the attractions and longer hours but that may not be what would be best for the bottom line.