Problem:
The help text for the --pidfilepath argument is misleading, this is what it says in 1.6.1

--pidfilepath arg directory for pidfile (if not set, no pidfile is
created)

However if you start mongo up as follows, you do not get a pid file but you do not get an error

shell> mongod --pidfile /foo/bar

test@alvin-ubuntu104-desktop:~$ sudo mongod --pidfilepath /data/db
Mon Aug 30 20:09:25 MongoDB starting : pid=30575 port=27017 dbpath=/data/db/ 32-bit
[...]
Mon Aug 30 20:09:25 db version v1.7.0-pre-, pdfile version 4.5
Mon Aug 30 20:09:25 git version: e2c5acbe4b511ca74b9db4ec556e6dc488263b2e
Mon Aug 30 20:09:25 sys info: Linux domU-12-31-39-01-70-B4 2.6.21.7-2.fc8xen #1 SMP Fri Feb 15 12:39:36 EST 2008 i686 BOOST_LIB_VERSION=1_37
Mon Aug 30 20:09:25 [initandlisten] waiting for connections on port 27017
Mon Aug 30 20:09:25 [websvr] web admin interface listening on port 28017

This argument needs to be a filename, so the correct syntax is

shell> mongod --pidfilepath /data/db/my.pid

test@alvin-ubuntu104-desktop:~$ ls -l /data/db/my.pid
rw-rr- 1 root root 0 2010-08-30 20:37 /data/db/my.pid

Solution:
a) if --pidfilepath is a directory, emit a warning message but continue to startup
b) change the argument description to say it needs to be a filename and not a directory

Reproduce:

• start mongo with a valid directory name
  shell> mongod --pidfilepath /foo/bar

Workaround:
Startup with a filename e.g.
shell> mongod --pidfilepath /for/bar/my.pid

Business Case:
User experience