I had a LaTeX document that was compiling without errors under both Windows and Ubuntu systems. When a friend tried to compile it on their Ubuntu system using pdflatex, it spewed this strange error:
! pdfTeX error (font expansion): auto expansion is only possible with scalable fonts
The error was strange because it gave no clue how to solve it. It turns out that the LaTeX installation on that particular Ubuntu system did not have scalable fonts. The common solution seems to be to install the cm-super package which has scalable fonts that can be used by LaTeX. On installing this package and recompiling the document, the error went away.
Tried with: Ubuntu 12.04.2 LTS