The usage formatter is brittle, especially the part that adds mutually exclusive markings to the normal usage string.  I don't think I've seen this error before, but I'm not surprised. 

A real fix requires a rewrite of the usage formatter, which I've suggested in an other bug/issue.  It would be a good idea to include this as a test case for such a patch.

The simplest immediate fix is to just not use a mutually exclusive group when it isn't needed.  I don't think it's urgent enough to require a special patch.

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start = actions.index(group._group_actions[0])

group is the mutually exclusive group that's being formatted.  _group_actions is is list of Actions.  The developer expected that the group would be populated (normally by 2 or more Actions), and didn't think it necessary to first check if it was empty.  That's a reasonable assumption.