Mission Impossible: QA Estimation Failed

Let's discuss the whole truth about Estimation (calculation of time) for the QA process in Agile. Here is how you can calculate how much time will be spent for analysis and testing (planning, writing, executing) when the business requirement may change or the technical solution is not yet fully thought out or something that will go wrong. Agile assumes be ready for the flexibility to change, how can you calculate the time for it for earlier? The team of the hero of my report also encountered this complex reality, but any impossible missions can be possible. How? I will tell you about it in my talk.