.travis.yml has

git:
  depth: 1

This means that if a another merge happens before a travis build is complete, the subjobs in that build will start breaking. I noticed this more lately due to the increase in PR's.

depth: 1 was added a few months ago to speed up builds. Maybe we should make it depth: 6 or similar, to allow a few closely-spaced merges? Or maybe we don't care, because the merge build will get superseded anyway by the next merge?

@arturo182
@tannewt