trigger myTrigger on myObject__c (before update, after update){
    for(myObject__c myRec :trigger.new){
        // This line will result in a 'record is read-only' error because
        //   it can be executed in an 'after' trigger context and attempts
        //   to update a record which is read-only at that point
        myRec.someField__c = 'newvalue';
    }
}