I have reproduced this problem with spurious error messages. It appears to be caused by the fonts used in your Input cell (recreating the cell with the default fonts does not produce the error messages). I also note that this appears to be related to dynamic highlighting - turning dynamic highlighting off in the Preference menu (using the appropriate checkbox in the Interface tab) also resolves the problem.
I have filed a report with our developers on this issue so that the problem can be fixed in a future version of Mathematica.
to which I responded:
Thank you for investigating this issue. I have a comment and a conclusion:
It is true that in the file I sent you, the font was smaller than normal. This happened during my efforts to reproduce the bug in a simple case. However the bug does occur in the "normal" font, provided that **some** of the text in the usage-string is reduced in size (by selecting it and pressing command-minus.) Reducing text like this is normal -- I learned to do it by copying the style Mathematica employs for its own usage statements.
For example, if you evaluate "?Part", you will see that Mathematica's usage statement for 'Part' contains both italicized and size-reduced portions -- and this evokes the bug. Therefore the bug is NOT caused by a "non-standard" font; it is caused by some change in v9 which broke Mathematica-style usage statements (when written by users). Therefore the bug might be deemed to have higher priority than otherwise.
I have thousands of such usage statements in hundreds of notebooks in dozens of projects -- and I learned the style by copying Wolfram!! With version 9, I have learned to ignore the errors in the Messages window, as they are 99.99% erroneous -- they appear by the dozens as I edit any of my many notebook files.