It is certainly possible to observe real conformations of amino acids, especially Arg, that are not quite common enough to have been identified as rotamers. To this point, note that defined rotamers accounted for only 82% of Arg conformations in the highly validated data set used to generate the MolProbity rotamer database.
http://kinemage.biochem.duke.edu/databases/rotamer.php#soPRL
If your Arg conformation fits your electron density, and does not have any internal or external steric clashes the conformation is likely real. As examples like yours are added to the database more rotamers may be defined!
Best,
-bob
