In text/plain notification e-mails, the actual event that took place is missing (unless it is a comment). This means that you get e-mails like the following:
--- you can reply above this line --- Issue 8344: "Something went wrong" when trying to connect accounts due to existing connection (BB-9629) https://bitbucket.org/site/master/issue/8344/something-went-wrong-when-trying-to Nicolas Venegas: --
This makes e-mail notifications pretty much useless for those of us who cannot/do not wish to read HTML email, because you have to go and see the issue to work out what happened anyway.
So far I have seen this happen with assignments and status changes.