I was using YT when some error messages appeared; since this happened immediately after I updated yt, I tried to "hg update 13651" to an earlier version.

It turned out to be a big mistake -- suddenly every command I tried (yt; yt update; hg update; and yt notebook) gave the following error:
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Traceback (most recent call last):
  File "/home/hao/projects/yt/bin/yt", line 9, in <module>
    load_entry_point('yt==3.0dev', 'console_scripts', 'yt')()
  File "/home/hao/projects/yt/lib/python2.7/site-packages/distribute-0.6.32-py2.7.egg/pkg_resources.py", line 337, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/home/hao/projects/yt/lib/python2.7/site-packages/distribute-0.6.32-py2.7.egg/pkg_resources.py", line 2311, in load_entry_point
    return ep.load()
  File "/home/hao/projects/yt/lib/python2.7/site-packages/distribute-0.6.32-py2.7.egg/pkg_resources.py", line 2017, in load
    entry = __import__(self.module_name, globals(),globals(), ['__name__'])
  File "/home/hao/projects/yt/src/yt-hg/yt/utilities/command_line.py", line 19, in <module>
    from yt.mods import *
  File "/home/hao/projects/yt/src/yt-hg/yt/mods.py", line 50, in <module>
    from yt.data_objects.api import \
  File "/home/hao/projects/yt/src/yt-hg/yt/data_objects/api.py", line 13, in <module>
    from grid_patch import \
  File "/home/hao/projects/yt/src/yt-hg/yt/data_objects/grid_patch.py", line 25, in <module>
    from yt.data_objects.data_containers import YTFieldData
  File "/home/hao/projects/yt/src/yt-hg/yt/data_objects/data_containers.py", line 31, in <module>
    from yt.data_objects.derived_quantities import GridChildMaskWrapper
  File "/home/hao/projects/yt/src/yt-hg/yt/data_objects/derived_quantities.py", line 26, in <module>
    from yt.utilities.parallel_tools.parallel_analysis_interface import \
  File "/home/hao/projects/yt/src/yt-hg/yt/utilities/parallel_tools/parallel_analysis_interface.py", line 29, in <module>
    from yt.utilities.lib import \
  File "/home/hao/projects/yt/src/yt-hg/yt/utilities/lib/__init__.py", line 21, in <module>
    from .geometry_utils import *
  File "geometry_utils.pyx", line 22, in init yt.utilities.lib.geometry_utils (yt/utilities/lib/geometry_utils.c:10907)
ImportError: cannot import name YTDomainOverflow
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