The window moves backward

Which string makes this longest unique substring calculation overestimate its answer?

if (last.containsKey(c)) {
    left = last.get(c) + 1;
}
last.put(c, right);
best = Math.max(best, right - left + 1);

Assume ordinary supporting code and valid inputs unless the question states otherwise.

Choose an answer before revealing the trace. Keys 1 to 3 work too.