When debugging  bug 711400  I found that bots that are configured as builder+tester combons can produce incorrect blame lists for the tester in case the builder has had a compile failure that caused some builds not being produced:

Because the builder was hitting compile failures in two builds [1] and [2], the tester didn't get the proper blame list (changes from between [3] and [4]) showing up in the Buildbot UI. It only showed two changes while the true blame list (if you look at the got_revision properties is:
https://chromium.googlesource.com/chromium/src.git/+log/d3916f29a4b748b20d9de39d8655dc2c8c29e190..e87bbc88306d3bbac6249887d846077c8d0f923b (i.e. 2 more commits than the currently displayed blame list at [4]).

The real blame list for the tester should have been from the last green builder build [5] and the next green one [6].

[1]: https://build.chromium.org/p/chromium.webrtc/builders/Mac%20Builder/builds/70692
[2]: https://build.chromium.org/p/chromium.webrtc/builders/Mac%20Builder/builds/70693
[3]: https://build.chromium.org/p/chromium.webrtc/builders/Mac%20Tester/builds/66924
[4]: https://build.chromium.org/p/chromium.webrtc/builders/Mac%20Tester/builds/66925
[5]: https://build.chromium.org/p/chromium.webrtc/builders/Mac%20Builder/builds/70691
[6]: https://build.chromium.org/p/chromium.webrtc/builders/Mac%20Builder/builds/70694