What is the biggest mistake you have made on a project and what did you learn from it?

I am not sure if it is the worst mistake, but once I was managing a project which had Sponsor A. Then, this person got transferred to another Business Unit so I assigned a new project sponsor (Sponsor B) WITHOUT letting B know about it. To make matters worse, the project was running over budget and was facing potential major delays. It goes without saying that I should have had a f2f meeting with the new Sponsor prior to taking any other action.

I worked at a smaller company one of the smaller departments and the whole organization was heavily silo'd with no PMO and no project standards. I was tasked with researching, budgeting, planning, managing, implementing, testing, commissioning, and all the administrative work that went along with it. With all of those plates spinning at once, and having to keep them all spinning, I did not perform due diligence on one of the supplier quote and passed it through for approval. Because of this, the approved budget was about 40% less than needed and I had to go back to our CFO with my hat in my hand, admit my mistake, and ask for quite a bit more money (more than I made in several years of employment).

Embarrassing and almost career ending (at that company) I have never accepted anything at first glance since then and that double and triple check mentality has served me well ever since. Reagan's adage "Trust, but verify." are PM words to live by.