The Slony upgrade procedure for minor versions can collide with concurrently running applications, even with read only transactions.

During the upgrade the stored procedures to upgrade the schema configure all log and deny access triggers. This is done with an "ALTER TABLE ... ENABLE/DISABLE TRIGGER ..." command. This command requires an access exclusive lock on the table, which conflicts even with read only access.

This command could be skipped if the trigger in question is already configured in that enable/disable state, which can be determined by a lookup in pg_trigger. I believe that during a pure minor version upgrade, none of the triggers actually needs to be reconfigured since they all should be in the desired state already.