On at least Ubuntu 11.x and Fedora 16, when using the HPLIP drivers and a LaserJet 2605 series printer (mine is a 2605dn) some characters will print as open square boxes, when printing documents created by a "text mode" application. This seems to be a problem with the PostScript interpreter in these printers when interpreting PS Level 3.
There may be another work around:
Open the file:
with a root-permissioned text editor. About line number 70, in the section named "Basic Device Capabilities", find the line that reads:
*LanguageLevel: "3" ` and change the "3" to "2". Save and restart CUPS:
$ sudo /etc/init.d/cups restart [or] $ sudo service cups restart
` and test. This work-around mitigates the problem...