Since v6.14 up until v6.17 (2010-05-12) tcsh interpreted strings with leading zeros as octal numbers, causing script breakage in our environment. Several users have reported this. See one example:

http://mx.gw.com/pipermail/tcsh/2010-January/004047.html

Simple test to illustrate the problem:

$ lsb_release -d
Description:    CentOS release 5.10 (Final)
$ tcsh --version
tcsh 6.14.00 (Astron) 2005-03-25 (x86_64-unknown-linux) options wide,nls,dl,al,kan,sm,rh,color,filec
$ tcsh
$ echo $0
tcsh
$ @ x = 5 + 08
@: Badly formed number.
$

$ lsb_release -d
Description:	Red Hat Enterprise Linux Server release 6.5 (Santiago)
$ tcsh --version
tcsh 6.17.00 (Astron) 2009-07-10 (x86_64-unknown-linux) options wide,nls,dl,al,kan,sm,rh,color,filec
$ tcsh
$ echo $0
tcsh
$ @ x = 5 + 08
@: Badly formed number.
$ 

An upstream patch to retain backwards compatibility has been added to v6.17 on 2010-05-12.

http://bugs.gw.com/print_bug_page.php?bug_id=97

tcsh 6.17 release 24 shipped in RHEL 6.5 is apparently based on the code from 2009-07-10 and doesn’t include this patch:

$ ls
tcsh-6.17-24.el6.src.rpm
$ rpm -qip tcsh-6.17-24.el6.src.rpm 
Name        : tcsh                         Relocations: (not relocatable)
Version     : 6.17                              Vendor: Red Hat, Inc.
Release     : 24.el6                        Build Date: Tue 18 Dec 2012 08:23:44 AM NZDT
Install Date: (not installed)               Build Host: ppc7.build.bos.redhat.com
Group       : System Environment/Shells     Source RPM: (none)
Size        : 949907                           License: BSD
Signature   : RSA/8, Wed 23 Jan 2013 05:19:15 AM NZDT, Key ID 199e2f91fd431d51
Packager    : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>
URL         : http://www.tcsh.org/
Summary     : An enhanced version of csh, the C shell
Description :
Tcsh is an enhanced but completely compatible version of csh, the C
shell.  Tcsh is a command language interpreter which can be used both
as an interactive login shell and as a shell script command processor.
Tcsh includes a command line editor, programmable word completion,
spelling correction, a history mechanism, job control and a C language
like syntax.
$ mkdir tcsh-6.17 && cd tcsh-6.17 && rpm2cpio ../tcsh-6.17-24.el6.src.rpm | cpio -id && tar -xzf *.tar.gz && { grep -Ri parseoctal * || echo "no match"; }
1864 blocks
no match
$ 

Was this done intentionally (e.g. to retain consistency in tcsh behavior between RHEL 5 and 6) or are you planning to include this particular upstream patch in the next release of tcsh for EL6?

tcsh 6.18.01 shipped with RHEL 7.0 Beta clearly includes this patch:

$ cat /etc/redhat-release 
Red Hat Enterprise Linux Everything release 7.0 Beta (Maipo)
$ tcsh --version
tcsh 6.18.01 (Astron) 2012-02-14 (x86_64-unknown-linux) options wide,nls,dl,al,kan,rh,color,filec
$ tcsh 
$ echo $0
tcsh
$ @ x = 5 + 08
$ echo $x
13
$