DocumentSourceUnwind does not advance properly if getCurrent() is not called before advance(). And there may be a problem with eof() if pUnwindValue is set but pUnwinder is exhausted.

Observed behavior: An incorrect result set may be generated for pipelines involving $unwind, particularly if used in conjunction with $skip.
Expected behavior: Correct result set.

Test:

c = db.c;
c.drop();

c.save( { a:[ 1, 2 ] } );
// One result expected, but none returned.                                              
printjson( c.aggregate( { $unwind:'$a' }, { $skip:1 } ) );