https://launchpad.net/~bzr/+archive/daily/+build/2891587 failed and the web page has no information about why it failed.
buildd-manager.log has the following:
2011-10-31 13:35:09+0000 [QueryProtocol,client] Asking builder on http://einsteinium.ppa:8221/filecache to ensure it has file bzr-git_0.6.2+git1425~ppa95~oneiric1.tar.gz (http://launchpadlibrarian.net/84126087/bzr-git_0.6.2%2Bgit1425%7Eppa95%7Eoneiric1.tar.gz, 194883274d1f8c0a647c75dc0e82f83a58eb1241)
2011-10-31 13:35:09+0000 [QueryProtocol,client] Asking builder on http://einsteinium.ppa:8221/filecache to ensure it has file bzr-git_0.6.2+git1425~ppa95~oneiric1.dsc (http://launchpadlibrarian.net/84126088/bzr-git_0.6.2%2Bgit1425%7Eppa95%7Eoneiric1.dsc, 1ba3c9012f0bf0510c06b5b9573c976cc5185fcb)
2011-10-31 13:35:18+0000 [QueryProtocol,client] Asking builder on http://americium.ppa:8221/filecache to ensure it has file chroot-ubuntu-lucid-amd64.tar.bz2 (http://launchpadlibrarian.net/51946824/chroot-ubuntu-lucid-amd64.tar.bz2, ac07c687a15a0d7295406dd36d280ec0e7616d49)
2011-10-31 13:35:19+0000 [QueryProtocol,client] Asking builder on http://nannyberry.ppa:8221/filecache to ensure it has file chroot-ubuntu-precise-i386.tar.bz2 (http://launchpadlibrarian.net/83295968/chroot-ubuntu-precise-i386.tar.bz2, c150837007dee27789cfba3dba7a65f20d415c79)
2011-10-31 13:35:19+0000 [QueryProtocol,client] einsteinium (http://einsteinium.ppa:8221/):
2011-10-31 13:35:19+0000 [QueryProtocol,client] ***** RESULT *****
2011-10-31 13:35:19+0000 [QueryProtocol,client] {u'bzr-git_0.6.2+git1425~ppa95~oneiric1.dsc': u'1ba3c9012f0bf0510c06b5b9573c976cc5185fcb', u'bzr-git_0.6.2+git1425~ppa95~oneiric1.tar.gz': u'194883274d1f8c0a647c75dc0e82f83a58eb1241'}
2011-10-31 13:35:19+0000 [QueryProtocol,client] {'archive_private': False, 'build_debug_symbols': False, 'archive_purpose': 'PPA', 'arch_tag': u'i386', 'archives': [u'deb http://ppa.launchpad.net/bzr/daily/ubuntu oneiric main', u'deb http://ppa.launchpad.net/testing-cabal/archive/ubuntu oneiric main', u'deb http://ftpmaster.internal/ubuntu oneiric main restricted universe multiverse', u'deb http://ftpmaster.internal/ubuntu oneiric-security main restricted universe multiverse', u'deb http://ftpmaster.internal/ubuntu oneiric-updates main restricted universe multiverse'], 'ogrecomponent': u'main', 'suite': u'oneiric', 'arch_indep': True}
2011-10-31 13:35:19+0000 [QueryProtocol,client] BuilderStatus.BUILDING: 7c25df41751a00397082f0a3f2f4f499bf1ca534
2011-10-31 13:35:19+0000 [QueryProtocol,client] ******************
2011-10-31 13:35:19+0000 [QueryProtocol,client]
2011-10-31 13:35:24+0000 [QueryProtocol,client] Starting factory <HTTPDownloader: http://litembilla.ppa:8221/filecache/buildlog>
2011-10-31 13:35:28+0000 [QueryProtocol,client] Asking builder on http://nannyberry.ppa:8221/filecache to ensure it has file bzr-git_0.6.2+git1425~ppa95~precise1.tar.gz (http://launchpadlibrarian.net/84126093/bzr-git_0.6.2%2Bgit1425%7Eppa95%7Eprecise1.tar.gz, 164f17bc112c9b92852897636421b02b7ea0ba01)
2011-10-31 13:35:28+0000 [QueryProtocol,client] Asking builder on http://nannyberry.ppa:8221/filecache to ensure it has file bzr-git_0.6.2+git1425~ppa95~precise1.dsc (http://launchpadlibrarian.net/84126094/bzr-git_0.6.2%2Bgit1425%7Eppa95%7Eprecise1.dsc, 0745f048616886cc7b7e0dfb4e1de48383619a5e)
2011-10-31 13:35:31+0000 [QueryProtocol,client] Asking builder on http://lychee.ppa:8221/filecache to ensure it has file chroot-ubuntu-natty-amd64.tar.bz2 (http://launchpadlibrarian.net/75435539/chroot-ubuntu-natty-amd64.tar.bz2, 2557c3a78b142ce8d001bc8a168870d855c2233c)
2011-10-31 13:35:35+0000 [QueryProtocol,client] nannyberry (http://nannyberry.ppa:8221/):
2011-10-31 13:35:35+0000 [QueryProtocol,client] ***** RESULT *****
2011-10-31 13:35:35+0000 [QueryProtocol,client] {u'bzr-git_0.6.2+git1425~ppa95~precise1.tar.gz': u'164f17bc112c9b92852897636421b02b7ea0ba01', u'bzr-git_0.6.2+git1425~ppa95~precise1.dsc': u'0745f048616886cc7b7e0dfb4e1de48383619a5e'}
2011-10-31 13:35:35+0000 [QueryProtocol,client] {'archive_private': False, 'build_debug_symbols': False, 'archive_purpose': 'PPA', 'arch_tag': u'i386', 'archives': [u'deb http://ppa.launchpad.net/bzr/daily/ubuntu precise main', u'deb http://ppa.launchpad.net/testing-cabal/archive/ubuntu precise main', u'deb http://ftpmaster.internal/ubuntu precise main restricted universe multiverse', u'deb http://ftpmaster.internal/ubuntu precise-security main restricted universe multiverse', u'deb http://ftpmaster.internal/ubuntu precise-updates main restricted universe multiverse'], 'ogrecomponent': u'main', 'suite': u'precise', 'arch_indep': True}
2011-10-31 13:35:35+0000 [QueryProtocol,client] BuilderStatus.BUILDING: 83b62c87c855b75a49bdb8b7fd6de55c7cc15f2e
2011-10-31 13:35:35+0000 [QueryProtocol,client] ******************
2011-10-31 13:35:35+0000 [QueryProtocol,client]
This happens when the build fails several times in succession without the build even starting, if the builders are broken or there's a missing chroot, etc.