A user can run simultaneously jobs which can merge PRs and read PRs from a bitbucket.
So this can turn into a race condition.
job A read list of the PRs and start iterating over all PRs to get status/details etc.. but job B running in parallel is merging a PR from the list. In this case job A will get 409 http error as the PR is out-of-date as documented here https://docs.atlassian.com/bitbucket-server/rest/5.16.0/bitbucket-rest.html#errors-and-validation