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I discovered this while programming the command line interface for a python program that can take a passed argument and throw it into the 'where like' clause of a SQL expression (intended for a postgresql database).
The wildcard character for where-like statements is generally the percent sign, which is how I found this ("WHERE %s LIKE '%--value%')".
If you use any single '%' signs in an ArgumentParser.new_argument(help=)'s help description Python 3.2 will throw an error.
Workaround: You can avoid this issue by doubling up on all % signs that you want to display in your help text.
parser.add_argument(('--foo', action='store',help='%bar') throws an error.
parser.add_argument(('--foo', action='store',help='%%bar') displays '--foo FOO %bar'.
Suggested fix:
When assigning help strings from add_argument(), throw them through a sanitizer and replace all occurrences of '%' with '%%' behind the scenes.
Example code (argparseBug.py):
from argparse import ArgumentParser
parser = ArgumentParser()
parser.add_argument('--foo', action='store', help='%bar')
args = parser.parse_args('-h'.split())
You get the following stacktrace:
Traceback (most recent call last):
 File "/path/to/script/argparseBug.py", line 6, in <module>
 args = parser.parse_args('-h'.split())
 File "/usr/lib/python3.2/argparse.py", line 1701, in parse_args
 args, argv = self.parse_known_args(args, namespace)
 File "/usr/lib/python3.2/argparse.py", line 1733, in parse_known_args
 namespace, args = self._parse_known_args(args, namespace)
 File "/usr/lib/python3.2/argparse.py", line 1939, in _parse_known_args
 start_index = consume_optional(start_index)
 File "/usr/lib/python3.2/argparse.py", line 1879, in consume_optional
 take_action(action, args, option_string)
 File "/usr/lib/python3.2/argparse.py", line 1807, in take_action
 action(self, namespace, argument_values, option_string)
 File "/usr/lib/python3.2/argparse.py", line 994, in __call__
 parser.print_help()
 File "/usr/lib/python3.2/argparse.py", line 2331, in print_help
 self._print_message(self.format_help(), file)
 File "/usr/lib/python3.2/argparse.py", line 2305, in format_help
 return formatter.format_help()
 File "/usr/lib/python3.2/argparse.py", line 279, in format_help
 help = self._root_section.format_help()
 File "/usr/lib/python3.2/argparse.py", line 209, in format_help
 func(*args)
 File "/usr/lib/python3.2/argparse.py", line 209, in format_help
 func(*args)
 File "/usr/lib/python3.2/argparse.py", line 515, in _format_action
 help_text = self._expand_help(action)
 File "/usr/lib/python3.2/argparse.py", line 601, in _expand_help
 return self._get_help_string(action) % params
ValueError: unsupported format character 'b' (0x62) at index 1

This is because the help text support substitution, as mentioned here: http://docs.python.org/dev/library/argparse.html#help
It's possible this documentation could be improved.

In case I wasn't clear, I mean that the help string supports %-formatting variable expansion, such as "%(default)s". I think it would be good if a sentence were added to the end of the "help" section, saying something like:
Because the help string supports %-formatting, if you want a literal "%" to appear in the help string, you must escape it as "%%".

That would probably work too.

Eric's suggested doc fix looks good to me.

New changeset c696416eb4e9 by Senthil Kumaran in branch '3.2':
Issue #13685 - Update argparse help message for % sign usage.
http://hg.python.org/cpython/rev/c696416eb4e9
New changeset 493d58c3c57f by Senthil Kumaran in branch 'default':
merge from 3.2 - Issue13685
http://hg.python.org/cpython/rev/493d58c3c57f 

Senthil, would you mind porting the fix to 2.7? Thanks.