Samba escapes filenames which cannot be used under Windows, such as COM1, allowing files with these names to be accessed from a Windows machine.  However according to the list from Microsoft[1], there are some reserved names which Samba does not escape and thus those files cannot be accessed from Windows via Samba.

Samba escapes COM[1-3], but not COM[4-9].  It escapes LPT[1-3] but not LPT[4-9].

[1] http://msdn.microsoft.com/en-us/library/aa365247.aspx