ac / dc rating for potentiometers ?

[Haible Juergen]

This question may ba a stupid one - or not.
Some smaller pcb mount potentiometers are rated for a certain AC voltage
only,
other ones (slightly larger) are rated for some AC voltage or a *lower*
(!) DC voltage.
I also remember one sales guy of some distributor mentioning "yes, this
pot works
with DC, too, but not this other type".
So are there really potentiometers that have problems with DC ? Some
kind of
particle migration or similar strange stuff ?
Somebody must know this !!

JH.