"I’m getting following error when doing update from Mercurial"

• what do you mean by "doing update from Mercurial" ...?
• what are the specific commands you run to reproduce this error?
• what environment information can you provide?
  • OS version
  • mercurial version
  • more is better, though the above two are most important

what do you mean by "doing update from Mercurial" ...?

Go to Phabricator=>Diffusion=>Select Mercurial Repository=>Edit Repository=>click "update Now"

what are the specific commands you run to reproduce this error?

From Unix box where we have Phabricator and repository cloned I run

cd /var/repo/QGTF/
[root@phdevphab QGTF]# hg log --template '{node}\1{branch}\2' --rev 'head()'
abort: unknown revision 'head()'!

[root@phdevphab QGTF]# hg log --template '{node}\1{branch}\2' --rev 'tip'
2a67e4c310b5605504b0c24ded2868c81cf7652c
Command with 'head()' - fails, command with 'tip' - works

what environment information can you provide?

CentOS 6.5
uname -oa
Linux phdevphab.chicago.local 2.6.32-431.el6.x86_64 #1 SMP Fri Nov 22 03:15:09 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

Can you run this command on your server and show us the output?

$ hg --version

On the phabricator box:

hg --version
Mercurial Distributed SCM (version 1.4)

On the Mercurial Repository box:
hg --version
Mercurial Distributed SCM (version 2.2.2)

You're running Mercurial 1.4, which is about 5 years old (released Nov, 2009). Update to a modern version of Mercurial.

In CentOS "yum install mercurial" installs version 1.4 by default

I run following commands :
yum remove mercurial
wget http://pkgs.repoforge.org/mercurial/mercurial-2.2.2-1.el6.rfx.x86_64.rpm
rpm -ivh mercurial-2.2.2-1.el6.rfx.x86_64.rpm
hg --version
Mercurial Distributed SCM (version 2.2.2)

Now issue is resolved, works perfectly ! Phabricator support rocks (magnificent,superb) ! Issue was resolved on the fly. Awesome !!!