I would say this is a bug, but I can't really be sure :)

According to my reading of http://search.cpan.org/~jtbraun/Parse-RecDescent-1.967009/lib/Parse/RecDescent.pm#Rejecting_a_production, if you replace reject:$failed with the equivalent

<reject: defined $failed> { print "WHAT!\n"; exit }
[download]

The program will print "WHAT!" and exit

So it could be that its a bug, or it could be the gotcha http://search.cpan.org/~jtbraun/Parse-RecDescent-1.967009/lib/Parse/RecDescent.pm#1._Expecting_an_error_to_always_invalidate_a_parse

update: It probably is a bug, because the actual equivalent would return UNDEF on failure and not EXIT, and when I do that, the error propagates, so it is probably a bug in reject autohandler or something ... Hooray, I can't fix it :)

#!/usr/bin/perl --
use Parse::RecDescent;

my $grammar =
q(
myrule : <rulevar: local $failed>
myrule : 'stuff' mysubrule(?) <reject: defined $failed> { print "WHAT 
+($failed)!\n"; undef }
| <error>
mysubrule: 'ID' <commit> '[' ']'
| <error?> { $failed++ }
);


my $parser = Parse::RecDescent->new($grammar) or die "hi";
for my $text (
    "stuff ID something",
    "stuff  something",
){
    print "text => $text\n";
    my $tree = eval { $parser->myrule($text) };
    warn $@ if $@;
    use Data::Dump qw/ dd /; dd $tree;
}

__END__
text => stuff ID something
WHAT ()!

       ERROR (line 1): Invalid mysubrule: Was expecting '[' but found
                       "something" instead

       ERROR (line 1): Invalid myrule: Was expecting 'stuff'
undef
text => stuff  something

       ERROR (line 1): Invalid myrule: Was expecting 'stuff'
undef
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update: It think this might be related issue, Bug #62892 for Parse-RecDescent: failed subrules eat text