mixing repositoies

[Stuart]

Till now, I have avoided repository mixing problem
by using only things from fedora-extras.  But as I
continue my slow move from Micro$oft, I think with
fc6 I will need to add some packages not available
in fc extras.  

I have seen lots of admonitions about the dangers
of mixing packages from, for example, livna and
dries.  But I have not seen any clear technical 
descriptions of exactly *what* (with examples) the 
problem is!  The stuff I have turned up with Google 
says only "Don't do it.  Just install this rpm (which 
will cause you to use my repo.)"

Can anyone point me to (or give me) a good detailed
technical explanation of the problems?  Has anything 
changed with fc6?