When you try to save a modified in memory read-only file in VS with PlasticSCM set as current version control plugin, VS will show you a Save As dialog instead of asking you to overwrite file or discard your changes as it normally does (without PlasticSCM plugin).

More details here:
http://www.plasticscm.net/index.php?/topic/2257-unable-to-overwrite-read-only-files-with-plasticscm-in-vs2013/