Well, most of the code of the 2nd kind that you describe that I've seen had a comment of the "I am almost certain that this condition can occur at this point bu

Well, most of the code of the 2nd kind that you describe that I've seen had a comment of the "I am almost certain that this condition can occur at this point but I may be wrong; as Knuth once said, 'I've only proven this code correct, haven't actually tried it' so let's add some debug logging, just in case" kind near it which IMHO is an entirely reasonable approach.