The Edit tool (and similar find-and-replace based code editing tools) fails or produces wrong results when you attempt to modify a file without first reading its contents. Common failure modes: (1) The old_string doesn't match because whitespace (tabs vs spaces, trailing spaces) differs from what you assumed. (2) The old_string matches multiple locations in the file, causing an ambiguity error. (3) The surrounding context has changed since you last saw it (another edit modified nearby lines). (4) Line endings differ (CRLF vs LF). (5) The indentation level is wrong because you guessed the nesting depth. (6) Unicode characters or special characters in the file don't match your assumed content. This wastes time with failed edit attempts and can corrupt code if a partial match succeeds at the wrong location.