Failed twice today with beta, es-ES and he, on OSX 10.7. There are similar bugs for the addons testrun - bug 957521 and in the eula test - bug 999385 but that failed just once in April so it might not be related.
Do we know which window that is? Maybe we could add more debuggable info to mozmill itself.
Here's a recent report: http://mozmill-release.blargon7.com/#/functional/report/bfd52839a01858de2f33d25f76948ac0 Only OSX 10.7.5 seems to be affected
Mozmill tests have been superseded by Marionette tests.