I went in and copied the thread text which the user claimed to cause the error and received the same result.
After slicing and removing text from the thread, I determined that the following statement was causing the error.
If I try to post a new thread which this simple word combo "System (" it trips a 404 error.
Seeing that the error isn't produced here when I post this thread, I have to assume this is a configuration issue? Perhaps mod_security or something? Any way to see what other phrases are kicking off this error? Assuming that I am hearing about this daily, I have to assume there are multiple causes.
I'd appreciate and be thankful for any leads!