When a Single Line Crashed Our Prod: A Real OOM Loop Bug Postmortem

Innocuous Code, Major Incident

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In a recent production deployment of our Spring Boot service, we added what looked like a harmless utility method during a routine refactor. The PR passed tests, got approved, and shipped. But within hours, alerts started firing — memory usage spiked, pods were restarting, and the system ground to a halt.

Let’s walk through what happened.

The Buggy Code

public List<String> getValues(String input) {
    List<String> result = new ArrayList<>();
    while (input != null) {
        result.add(input);
        input = input.trim(); // Looks innocent
    }
    return result;
}

What’s the Output?

If we run getValues(" Hello "), what do we expect?

• trim() removes whitespace.
• But input = input.trim() on a string like "Hello" returns the same object.
• Now the condition while (input != null) never breaks, because input remains non-null and unchanged — leading to an infinite loop.
• Add .add(input) inside, and memory usage explodes: every…