odd hg log behavior?

[j w]

I'm going through the 'hello' example in hgbook.pdf
I cloned the repo, now looking at 'hg log', and got this odd behavior

C:\hello>hg log -r 5:0
abort: unknown revision '5'!                                     // as expected

C:\hello>hg log -r 8:0
changeset:   1:82e55d328c8c                                // huh?
user:        mpm at selenic.com
date:        Fri Aug 26 01:21:28 2005 -0700
summary:     Create a makefile

changeset:   0:0a04b987be5a
user:        mpm at selenic.com
date:        Fri Aug 26 01:20:50 2005 -0700
summary:     Create a standard "hello, world" program


Why does specifying 8:0 behave like that?
I tested this on Windows with 0.9.5, and FreeBSD with 0.9.4, both
behave the same in this respect.

-J