Your question is Diagnose Bad Data in Pipelines. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You're supporting a data pipeline that feeds customer-facing workflows, and a customer reports behavior that looks wrong in the product. Before treating it as an application bug, you want to determine whether the issue comes from incorrect, missing, duplicated, or delayed data moving through the pipeline.
What would you do if you suspected a customer issue was caused by bad data rather than a product bug?