What version of CssMin are you using (source and/or build)?
3.0.1

What was the input stylesheet and/or configuration options?
background-position: 0 0; + plugin CompressUnitValues true

What is the expected result?
background-position: 0 0;

And what is the actual result and/or error message?
background-position:0; which uses the default (center) as second parameter 
(background-position: 0 center;) - tested with Firefox

Please provide any additional information below.
This behavior should be fixed for background-position! Never use just one 
parameter in background-position!