A.E.Housman (probably, or at least possibly) once wrote a couplet to follow Wordsworth's lines in 'To the Cuckoo':
O Cuckoo! Shall I call thee bird,
Or but a wandering voice?
State alternative preferred
With reasons for your choice.
Another four line poem is quoted by Keats in a letter, but doesn't seem to me to be claiming authorship of it. And where it crops up elsewhere, it's often as the punchline to a story in which an undergraduate calls on a friend, only to find he has gone out, but has left on his desk a pad on which he has written the first two lines of a new poem:
''The sun's perpendicular rays
Illumine the depths of the sea'
Which the visitor helpfully finishes off for him:
'The fishes, beginning to sweat,
Cry 'Damn it! How hot we shall be!''