TITLE: The unpublished papers in reference to the published papers
QUESTION [29 upvotes]: Sometimes it happens that a published paper refers to an unpublished paper for a result used.
In this case, if we want to check this result by ourselves, we need to access to this unpublished paper.  
Question: What is the usual process for accessing to an unpublished paper?
 Is it the duty of the author (citing it in his published paper) to send it to anyone requiring it?
Or is it the duty of the editor, or of the author of the unpublished paper?   
What to do if we can not get it through these means?

REPLY [4 votes]: As the reader, you can also ask mathoverflow for help finding the paper.  There have been some very interesting MO questions along this line.  In particular, this record helps others to also track down the same paper (or discover it never existed in the first place).  Feel free to add to this list since this is CW:

Scott-Solovay unpublished paper on ``Boolean valued models of set theory'' (never existed)
Looking for a copy of Leo Harrington's unpublished notes on the first nonprojectible ordinal (now we have a version online)
Harrington's unpublished note "The constructible reals can be anything" (now we have a version online)