Watch a category “foo”
Post a topic in a different category “bar”
Recategorize the new category to the category “foo”
The topic will not be watched.
Also works in the opposite direction: sometimes when things are erroneously posted to a meta category (which I am watching) and then recategorized to a different category (which I am not watching), it still shows up as watched for me. e.g. changing the category didn’t update the watch state.
But it should!