Most failed redactions look perfect. A black rectangle over a name hides it on screen and in print, and leaves the name sitting in the file where anyone can select it, copy it, or search for it.
If copying, searching or extracting images reveals the hidden content, redaction failed. Passing those checks alone does not prove removal: unused objects, form values, attachments and metadata can retain information too.
Removal can affect whole glyphs or annotations that overlap a cover. Inspect the saved document for unintended changes; PalPDF whiteout does not guarantee secure redaction.