If you happen to have Zope Interface 3.5.1 - which, I should note, is the version included with just about every release of OS X people are now using, you get this error:
File ".../twisted/application/reactors.py", line 12, in <module>
from twisted.plugin import IPlugin, getPlugins
File ".../twisted/plugin.py", line 34, in <module>
from twisted.python.modules import getModule
File ".../twisted/python/modules.py", line 68, in <module>
from twisted.python.filepath import FilePath, UnlistableError
File ".../twisted/python/filepath.py", line 555, in <module>
File ".../zope/interface/declarations.py", line 495, in __call__
raise TypeError("Can't use implementer with classes. Use one of "
TypeError: Can't use implementer with classes. Use one of the class-declaration functions instead.
It should instead say something like "You need Zope Interface at least 3.6 (but ideally earlier than 4.0 unless you want to see warnings...)" so the user knows what to fix.