Date bug. (BB-4468)
Take local date back, example any date on 2009year. Create commit. Push commit. Go to bitbucket and see what happened. I dont know, can yo reproduce this or not , but my repo have a strange rollback with some not used merges and other strange things.
And more info: Its maked some times ago, before this bug : Create repo. Make some numbers of commit. Wipe local repo. Push forced for applying changes and dotn loose commit history.
This is just git, i dont know about hg.