Hello
In the Opportunity object, I created three record types and two custom fields:
- a picklist "Opportunity Sub-type" controlling 
- a picklist "Funnel Stages" with in fact exactly the same values as the picklist standard field "StageName"
The purpose is to limit in the different record types the stages to use for each Opportunity Sub-Type.
As the field StageName is mandatory on Opportunity and not possible to define field dependencies with, I created a trigger to sync the StageName value when the same one is selected in the "Funnel_Stages__c" picklist.
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trigger UpdateSource on Opportunity (after update) 
{                                                                      
  List<Opportunity> opportunitiesToUpdate = new List<Opportunity>();     
  for(Opportunity currOpportunity : trigger.new) 
  {
    currOpportunity.StageName= currOpportunity.Funnel_Stages__c;          
    leadsToUpdate.add(currOpportunity);                                       
  }
  update opportunitiesToUpdate;                                        
}
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Not error when compiled but well when activated as I had thefollowing error message on the Opportunity page:
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Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger UpdateSource caused an unexpected exception, contact your administrator: UpdateSource: execution of AfterUpdate caused by: System.FinalException: Record is read-only: Trigger.UpdateSource: line 6, column 1
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Can somebody help me?
Thanks already.
Best regards
Jean-Luc