Find the Most Common Mistake

Duolingo's review-scheduling service decides how many days until a lesson comes back for review, and tracks each learner's correct-answer streak. This is the class an interviewer showed a candidate.

public class ReviewScheduler {
    static Map<Long, Integer> streaks = new HashMap<>();

    public int computeNextIntervalDays(long userId, int previousIntervalDays, int easeFactor) {
        int correctStreak = streaks.getOrDefault(userId, 0);
        correctStreak++;
        streaks.put(userId, correctStreak);

        if (correctStreak % 5 == 0) {
            return 0;
        }

        int nextInterval = previousIntervalDays * easeFactor;
        return nextInterval;
    }

    public boolean isDue(int daysSinceLastReview, int dueInDays) {
        return daysSinceLastReview < dueInDays;
    }
}

Find the mistakes in this scheduling logic and explain how you would correct each one, in free text.