The thirteen error classes

The error taxonomy. Thirteen classes cover nearly every lost point. Each has a signature symptom — learn the mapping and debugging becomes lookup:

1. Off-by-one index — exception or a missed end element.
2. Wrong loop boundary — one extra or one missing iteration.
3. Inverted boolean condition — exactly backwards behavior.
4. Wrong accumulator op — sum where count belongs (or minus where plus).
5. Unreset state — value from a previous run leaks into the next (streak counters, running totals).
6. String index error — charAt/substring past the ends.
7. ArrayList mutation-scan — forward removal skips the shifted element.
8. Object state error — a field mutated by the wrong method or read before assignment.
9. Constructor error — shadowed parameter, forgotten field.
10. Inheritance error — override missing super. or the override never called.
11. Polymorphism error — reference type read where the object type governs.
12. Recursion error — missing/wrong base case or no progress.
13. Missing edge case — works on the example, dies on empty / single / extremes.

Diagnosis habit: name the CLASS first, then find the line. The name tells you what to look for (an index? a flag? a base case?).