Is this a chromium-build.appspot.com bug/

Yes, but it is because the chromium.chromiumos master itself has two separate "default" categories:
https://build.chromium.org/p/chromium.chromiumos/console

I think chromium-build keeps track of the names of column sets, and with two sets having the same name, one overwrote the other in a dictionary.

I'm really curious how chromium.chromiumos got two separate groupings both called "default". I didn't think that was possible...

Deleted: Screenshot from 2016-06-28 15:20:02.png 2.6 KB

	Screenshot from 2016-06-28 15:20:02.png 2.6 KB View Download

Figured it out. This is just fallout from how mind-numbingly dumb the category system is. https://codereview.chromium.org/2109883002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build.git/+/a8fedd0fdc9df67ee5666ffe8908f424c4a11a1e

commit a8fedd0fdc9df67ee5666ffe8908f424c4a11a1e
Author: agable <agable@chromium.org>
Date: Wed Jun 29 00:20:08 2016

Fix multiple 'default' categories on chromium.chromiumos master

R=dnj@chromium.org
BUG= 624036 

Review-Url: https://codereview.chromium.org/2109883002

[modify] https://crrev.com/a8fedd0fdc9df67ee5666ffe8908f424c4a11a1e/masters/master.chromium.chromiumos/builders.pyl

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/infradata/master-manager.git/+/f91eb7bd05a8ddcc9422d1a6040127979562eb6c

commit f91eb7bd05a8ddcc9422d1a6040127979562eb6c
Author: agable <agable@google.com>
Date: Wed Jun 29 15:58:44 2016

Will be fixed this evening when the master restarts.