jenkins will sometimes claim that "This change was unable to be automatically merged with the current state of the repository." and vote V-1 when the change is mergeable. I've seen this happen a few times in the recent weeks, most recently on PS14 of https://gerrit.wikimedia.org/r/#/c/96810/ just now.
The change shows "Can Merge: Yes", forcing a recheck or rebasing via gerrit results in V+1/2.
Version: unspecified
Severity: major