If this is the issue for tracking the problem first reported in #8154, I think this bug is far from trivial! And still open five years later! (Maybe it has gotten worse in 5 years thanks to other changes.)
As it currently stands, if you make a commit using the online editor, it looks like you can activate a bookmark and have that commit advance the bookmark. But instead, what happens is that Bitbucket creates a new branch with the same name as the existing bookmark, and commits to that instead.
Since Mercurial branches are permanent aspects of the commit, this is very messy to clean up. (You need to graft the commit back onto the main branch from the command line and then strip the erroneous commit and any commits after it on both your local copy and on Bitbucket.)
PS, Feel free to copy this comment back to that more specific issue in #8154, if you unmark that issue as a duplicate of this one. I'm not sure what MattT was thinking of when this request was posted back in 2013, but it sounds like the highly-voted feature request in #6705 would cover it.