To follow up to myself …
The limit seems to be 1916 characters … anything longer than that, “poof”. Rather odd number.
The exact error message is:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, firstname.lastname@example.org and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
[line] Apache/2.0.63 (Unix) mod_ssl/2.0.63 OpenSSL/0.9.7a mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/220.127.116.1135 PHP/5.2.6 Server at http://www.thetankatwaterheaterrescue.com Port 80