Prior to WP 4.2, terms in different taxonomies with the same slug (for instance, a tag and a category sharing the slug “news”) shared a single term ID. Beginning in WordPress 4.2, when one of these shared terms is updated, it will be split: the updated term will be assigned a new term ID.
It sounds helpful. But indeed, it’ll be more complicated than it looks whenever have to face this. Luckily, until now, not facing one.
Anyway, just notice there is a Plugin Developer Handbook. Somehow need to allocate time to read this. Skimming at least.