I just have to say...
Had the same issue today. This thread got me thinking, so I broke up my content into smaller pieces and kept troubleshooting until I found the offender!
It was / - only little backslash character that caused me so much brain pain.
Another one was word "CD"...bewildered!
After I got rid of it, everything worked for me.
So thanks Ryan and hope this helps someone.
And - Pete - yes, the server has mod_security (which can cause trouble for developer)