A few weeks ago the Yellow Squad made another change to increase the relevance of the email you get from Launchpad by eliminating some noisy ones. A while back, Matthew Paul Thomas noticed that a change to a bug that was subsequently reverted could be deemed a mistake and was an action no one really needed to know about. As is his habit, mpt opened a bug (164196) with the suggestion that corrected actions not generate email.
So if Alice is assigned to a bug by mistake and then unassigned within five minutes no email is generated. Likewise, if a tag is added but then quickly removed the action does not cause any email to be sent.
Note that avoiding email requires that the action be undone, not just fixed. By that I mean the bug must be returned to the original state to be recognized as an undoing. So if you assign Alice to a bug by mistake and then change that assignment to Bob then the action will not be seen as a mistake. Email will be sent notifying about both assignee changes to Alice and then to Bob.