Our setup uses the private-fork model.
A build-bot scans open pull-requests, and tries to checkout the current 'fromRef' to run some automated Q/A.
Occasionally, it seems the refs/pull-requests/<X>/from ref isn't updated when the pull-request is updated. (I.E. new commits pushed to the from-branch of the pull-request). Neither is the new fromRef available in the toRef-repository. (Tested with direct commit-fetch)
In this case, the REST-API shows the updated commit-hash for the newly added commit, which cannot be fetched from the toRef-repository, which halts the checkout process.