Actually, it's not commonplace - at least not with "good" PDF files. This does point to a font issue, and given the symptom, I would assume that it's due to a problem with the PDF file.
Font issues can have many different causes. It could be that the application that created the PDF file did not correctly embed or reference the font, it could be that the font on your system is corrupt, and a few more things that could cause problems like this. It is also possible, that you came across a bug in Adobe Reader - even though much less likely than a problem with the PDF file.
The first thing I would try is to toggle the "Use local fonts" setting in Acrobat's or Reader's preference. You can find this setting in the "Page Display" category, in the "Rendering" group.
If this does not fix the problem, then try to figure out what fonts are used and if they render correctly in a different PDF file. To find out which fonts are used in a PDF file, open up the Document Properties dialog (Ctrl-D or Cmd-D) and go to the "Fonts" tab. Here you will find a list of all fonts, and if the are not embedded, subset embedded or fully embedded. You can then try to either find another PDF file that uses the same fonts, or create one (however, if you only have Adobe Reader, this is a bit more complicated).
Another thing to check is which application created this PDF file. You do this again on the "Document Properties" dialog, this time on the "Description" tab. If you do not find any information about the "PDF Producer", or if you find an application you've never heard of, it may be the PDF producer's fault.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com