libpam-modules dependency loop stable-->unstable

[Brian K McDonald <bkm@ece.pdx.edu>]

After a fresh "stable" install, after adding "unstable" lines to 
/etc/apt/sources.list, and performing the following sequence:

dselect update
dselect select (immediatetly Q[uit])
dselect install

I get the following:

E: This installation run will require temporarily removing the essential
package libpam-modules due to a Conflicts/Pre-Depends loop.  This is 
often bad, but if you really want to do it, activate the 
APT::Force-LoopBreak option.

There are posts on this error message going back at least three years.  

Any advice on how to deal with it, understand it, or prevent it?

Thanks,

bkm