TestCancelSave.cs

using System.Linq;
using Umbraco.Core;
using Umbraco.Core.Components;
using Umbraco.Core.Events;
using Umbraco.Core.Models;
using Umbraco.Core.Services;
using Umbraco.Core.Services.Implement;

namespace Umbraco.Web.UI
{
    [RuntimeLevel(MinLevel = RuntimeLevel.Run)]
    public class TestCancelSave : IUserComposer
    {
        public void Compose(Composition composition)
        {
            composition.Components().Append<TestCancelSaveComponent>();
        }
    }

    public class TestCancelSaveComponent : IComponent
    {
        public void Initialize()
        {
            ContentService.Saving += ContentService_Saving;
        }
        
        public void Terminate()
        {
        }

        private void ContentService_Saving(IContentService sender, SaveEventArgs<IContent> e)
        {
            var content = e.SavedEntities.First();
            
            if (content.Name.Contains("error"))
            {
                if (e.CanCancel)
                {
                    e.CancelOperation(new EventMessage("PreFlight", "There was a problem with some content", EventMessageType.Error));
                }
            }
        }
    }
}