Travis jobs reporting success, even though tests fail (using tox)

I'm looking specifically at the following build:
http://ift.tt/2wosv2N

All jobs seem to be reporting as successful, even though they all have a single, deliberately failing test, and this has been happening on different builds on the same project for at least the last 2 days.

The configuration in my .travis.yml hasn't changed significantly in a while, apart from switching to 'trusty' from 'precise' - and changing that back seems not to fix the issue.

My tox.ini hasn't been changed in a while either.

I tried forcing tox to an earlier version already, which didn't seem to help.

I know it's got to be something to do with tox or Travis, but that's where my knowledge ends. Any help at all would be greatly appreciated.