Run a tiny, safe input whose expected output you know. Inspect the value after each node, starting at the source. The first step where actual and expected values diverge is where to focus; later nodes may only be carrying the error forward.
Check type, encoding, missing fields, and branch conditions before rewriting the graph. Test an empty value and a forced failure. If the workflow touches an external service, distinguish a bad transformation from a permission error or timeout. Keep a copy of the previous working version while you change one step at a time.
For a forum report, include the first bad node and sanitized before-and-after values.
Akousa