Refresh Token always reported as "Unknown or Previously Used"

[Ted Benson]

Has anyone else had a problem refreshing MeetUp tokens?

I perform OAuth requests as per the docs and everything works great. But every time I attempt to refresh a users's token, I get the response 

{ error_description: 'Unknown or previously used refresh_token',

  error: 'invalid_grant' }

This happens even if I do the following steps in immediate succession:

1. Authorize a user over OAuth and get back token & refresh_token

2. Immediately make a refresh call

Any thoughts or similar experience? The code is so plain-vanilla OAuth that I've run out of creative ways to thinking about debugging attempts. 

Thanks very much!

Ted