Pilots are evaluated on curated inputs by people who understand the intent and unconsciously phrase things well. Production is the opposite: adversarial in the ordinary sense, full of edge cases, abbreviations, missing context and users who will not rephrase to help. The accuracy figure from the pilot does not survive that transition, and usually nobody measured it in a way that would have predicted this.
The second failure is retrieval, not generation. Most enterprise use cases are grounded in the organisation's own content, and the quality ceiling is set by whether the right passage can be found — which is a data engineering, permissions and content quality problem. Teams tune prompts for weeks when the actual defect is that the source document is out of date, badly structured, or that half the corpus is invisible to the user asking.
The third is that nobody defined what happens when the system is wrong. Not the error rate — the consequence. Who notices, how, what the user sees, whether the output can be traced to its sources, whether a human can intervene before an action is taken. A system that is right 95% of the time and has no answer for the other 5% is not deployable in a process that matters, and the 5% is where all the real design work lives.