I am having the same problem as this list member:

  http://www.mail-archive.com/slony1-general@gborg.postgresql.org/msg03382.html

I'm using Slony 1.2.14 on OS X (installed via MacPorts).

Here's a sample script:

  create set (id=10000, origin=1);
  set add table (set id=10000, origin=1, id=15, fully qualified
name='public.profiles');
  subscribe set (id=10000, provider=1, receiver=2, forward=yes);
  wait for event (origin=2, confirmed=1);
  sync (id=1);
  wait for event (origin=1, confirmed=2);
  merge set (id=1, add id=10000, origin=1);

This fails with:

<stdin>:4: PGRES_FATAL_ERROR select
"_origo_development_test".mergeSet(1, 10000);  - ERROR:  Slony-I: set
10000 has subscriptions in progress - cannot merge

There are no errors in the slon daemon logs.

Alexander.