Branch list sometimes shows all feature branches as thousands of commits ahead of main branch (BB-10643)

We just had a weird issue for a few minutes where the web UI showed our git master branch as having no commit history, and branches were thousand of commits ahead - https://bitbucket.org/grabone/gocoupons/branches

When I cloned from the repo I got a sensible looking checkout, and looking at the web UI now it's returned to normal - our branches are a few commits ahead/behind of master as expected.

WTF?