% symbol causes expception when parsing help text

[David Hayes]

Help fields containing a % symbol cause an exception when using the -h or --help flags to parse help text.

% is obviously a special symbol used by the parser for some reason, however there seems to be no documentation of this, or any other reserved symbols.

Exception text - http://pastebin.ubuntu.com/7250072/
Python 2.7.3
Argparse 1.1