What is the most difficult and challenging Agile scenario you've faced?

As mentioned in the Think Like an Expert post, I'm looking to create Think Like an Agilist, a way for people to practice their thought processes using challenging Agile scenarios.

Step one is to collect these scenarios.

So... think back and try to remember the hairiest, most difficult, most challenging Agile scenario you've had to deal with.  Think about the context, who was involved, what the stakes were, what you were thinking...