"From your second picture is looks almost like there was an existing crack..."
I agree. See the oxidation that has set in on part of the break? That crack has been there a while indicative of some past trauma. I've seen cranks that looked as if they had been raised from the Titanic that still worked just fine.
So....in answer to your question, you should replace your cranks every time they do that.
Last edited by slorollin; 06-07-14 at 08:33 AM.