Historically, two terms in different taxonomies with the same slug (for instance, a tag and a category sharing the slug "news") have 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.
In the vast majority of situations, this update will be seamless and uneventful. However, some plugins and themes store term IDs in options, post meta, user meta, or elsewhere. WP 4.2 will include two different tools to help authors of these plugins and themes with the transition.
When a shared term is assigned a new term ID, a new 'split_shared_term'
action is fired. Plugins and themes that store term IDs should hook to this action to perform necessary migrations. The documentation for the hook is as follows: