Most of the time we make tests when something is in our radar. I guess you have use case in your spec that tells you that's the expected behaviour, or at least

Most of the time we make tests when something is in our radar. I guess you have use case in your spec that tells you that's the expected behaviour, or at least some clever developer figured out it should be done that way. But it is not always so "easy": Sometimes you don't have the domain knowledge, or the spec doesn't say a word about negative cases...