Why doesn't it work?

Sometimes you are in a situation where you get requirements and it is all confirmed by end users and your business analyst. You head off merrily delegating tasks and breaking this requirement into smaller chunks and work on delivering the solution.  Once done, the end users say what is this? Why does it work like this? This is a classic example of a waterfall creeping into an agile world.

Things you can do to try to safeguard yourself

1. Get requirements early and reviewed with everyone in the team
2. Rapid prototyping. Everything is good in theory but sometimes when you see it in front of your face it is not exactly as you imagined. This is the same for business users. Fail early, fail fast and improve upon it to come up with a better solution.
3. Don't be afraid to show progress even if it is something small. Many small wins are better than one massive fail. If you were able to do something minor like being able to upload data, show people, this might lead to discussions of how it would be stored and data retention questions. 
4. Review and revise requirements with everyone in the team again at each demo stage

Make sure everyone on the team who is involved with the feature is 110% sure on how it works from end to end. Any grey areas will lead to ambiguous assumptions and that usually leads to confusion and maybe even failure.