using(SPSite site = new SPSite("http://portal")) 
{
    using(SPWeb web = site.OpenWeb())
    {
        web.AllowUnsafeUpdates = true;
            
    
        SPList list = web.Lists["TestCustomList"];
        
        // open any item in a simple custom list (100) with versioning turned on and no additional fields beside Title
        SPListItem item = list.GetItemById(1);
        
        // log the amount of versions and current value of the updated field
        Console.Write(string.Format("Current amount of versions: {0} - current Title: {1}", item.Versions.Count, item["Title"]));
            
        // change Title of the item
        item["Title"] = item["Title"] + " X" 
        
        // update the item - should create new version        
        item.SystemUpdate(true);
        
        // load the item again into a new variable
        SPListItem itemNew = list.GetItemById(1);
        
        // log the amount of versions and new value of Title
        // NO NEW VERSION WAS PRODUCED (tested across several simple custom lists with no event receivers, no alerts, nothing)
        Console.Write(string.Format("New amount of versions: {0} - new Title: {1}", itemNew.Versions.Count, itemNew["Title"]));
                              
    }
}