The version 3.2.1 of the acts-as-taggable-on gem has a nasty bug. When you remove a tag from a resource (let's say you have posts) it will remove the tag! So for example if you have two posts with a common tag and use tag_list method to change the tags in one of the post, it will delete the previous tags even if it is associated with two posts! I am not sure which versions affected, but in the last version (3.2.5) works correctly, without deleting the previous tags.
