I've noticed that there are spurious phpunit failures. A rebase, triggering a retest, tends to workaround the issue. I've given one that there does not seem to be a bug for. I don't know if there are others.
On the integration server, one example is https://integration.mediawiki.org/ci/job/mediawiki-core-phpunit-api/1029/console . The relevant excerpt is:
06:27:05 [exec] There was 1 error:
06:27:05 [exec] 1) ApiBlockTest::testBlockingActionWithNoToken with data set #1 ('unblock')
06:27:05 [exec] PHP_Invoker_TimeoutException: Execution aborted after 2 seconds
The relevant change is https://gerrit.wikimedia.org/r/#/c/41808/ . There is nothing PHP or API-related in the change.
It looks like the timeout is relatively short, but when it's exceeded, it fails the test. This is disruptive to people working on unrelated code.