I also have a midwest-ferret playpen and have had a similar problem with
the lower ladder being knocked down.  So far the only thing I've come up
with is to bend the ends of the ladder that go through the first shelf a
little so that they bend towards each other.  This adds a little more
tension so that the ladder doesn't slip out as easily.  Keep in mind that
this also makes it a little harder to put it back into place too.  I think
my younger, more hyperactive, male is responsible for rearranging the
ladder and he still manages to knock it down occasionally.  I just make
sure I check the ladder regularly and usually I catch it when it's still
hooked on one side.
 
If anyone else has any suggestions I'd like to hear about them too!
[Posted in FML issue 2728]