Code Debugger — illustrative example

INPUT
Python reproduction:
items = ["A", "B"]
seen = []
for i in range(len(items) - 1):
    seen.append(items[i])
Expected ["A", "B"]; actual ["A"].

RESULT
Corrected loop:
seen = []
for item in items:
    seen.append(item)

For ["A", "B"], seen becomes ["A", "B"]. Empty input produces []. Iterating over items directly removes the incorrect index boundary.

REVIEW
Debugging starts with expected behavior, actual behavior and a reproducible case. A likely cause should be tested rather than immediately declared the root cause.

This is a teaching example, not a live execution record.