This just happened to me in a nightly build:

GNU Emacs 24.3.50.1 (x86_64-apple-darwin, NS apple-appkit-1038.36)
 of 2013-03-20 on bob.porkrind.org

I get this error once in a while when editing C or C++ code. This time it
happened while  reading SICP chapter 5 and scribbling, source attached.
Unsure how to debug it.