CL (https://chromium-review.googlesource.com/c/407636/4) passed PRE-CQs without problems but failed strago-paladin BuildPackages 5 times. 

https://luci-milo.appspot.com/buildbot/chromeos/strago-paladin/5634
https://luci-milo.appspot.com/buildbot/chromeos/strago-paladin/5681
https://luci-milo.appspot.com/buildbot/chromeos/strago-paladin/5682
https://luci-milo.appspot.com/buildbot/chromeos/strago-paladin/5683
https://luci-milo.appspot.com/buildbot/chromeos/strago-paladin/6288

It turns out CL:407636 should depend on CL:431877 but it didn't add CL:431877 as CQ-DEPEND until patch set 5.

Both CL:407611 and CL:431877 marked CL:407636 as CQ-DEPEND, so when PRE-CQs got run on CL:407611 and CL:431877, PRE-CQs picked up CL:407636 in the same run. PRE-CQs passed without problems and marked all 3 CLs as Ready.(For example, https://luci-milo.appspot.com/buildbot/chromiumos.tryserver/pre_cq/19079)

But when CQ scanned CLs to test, CL:431877 wasn't marked as +1 Commit-Queue, so CQ only picked up CL:407636 and resulted in the BuildPackages error as above. (For example, https://luci-milo.appspot.com/buildbot/chromeos/strago-paladin/6288)