I use different speciaiisations of appointments. They each have there own ItemPresenter.
I subscribe to the event ItemPresenterCreated of the calendar and then set the corresponding Style depending on the type of presenter.
It appears that when navigating to another month some times an incorrect style is used by the control. In this example, on the 9th of march style A is used, however when navigating to april, style B should be shown on the 13th (same cell location as march), but incorrectly style A is used.
If I reload the calendar while staying at april, then style B is used correclty, but then style B is also used incorrectly when navigating back to march.

I am using the trial 2 version you provided me in another thread (Re: convert calendar into pdf).
It is not the style that is incorrect, but the ItemPresenter object. I have 2 different ItemPresenter classes.
I've overridden the OnItemChanged and indeed I noticed that an ItemPresenter of Type_A object is at some point associated with an Item of Type_B object. This should not happen. ItemPresenter Type_A should only be re-used for Items of Type_A.

Thanks, in the meantime I got another exception in the same area. When removing the recurrence of an item I get a NullReferenceException. Call stack:

bij MindFusion.Scheduling.Silverlight.ItemPresenter.OnItemChanged(Item oldItem, Item newItem)
bij Trainers.CalendarHelpers.TrainingPresenter.OnItemChanged(Item oldItem, Item newItem)
bij MindFusion.Scheduling.Silverlight.ItemPresenter.OnItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
bij System.Windows.DependencyObject.RaisePropertyChangeNotifications(DependencyPrope
rty dp, Object oldValue, Object newValue)
bij System.Windows.DependencyObject.UpdateEffectiveValue(DependencyProperty property, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, ValueOperation operation)
bij System.Windows.DependencyObject.SetValueInternal(DependencyProperty dp, Object value, Boolean allowReadOnlySet, Boolean isBindingInStyleSetter)
bij System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
bij MindFusion.Scheduling.Silverlight.ItemPresenter.set_Item(Item value)
bij A.c6d9dec63fbcdc1ec515a79b519234bee.UpdateItemPosition(Calendar calendar, c97e5673e0c605b2971b80c00b1256bf7 viewCell, Object parameter, Boolean timetableAllDay, c344a63b1287e10df4eee7c38a9892791 type)
bij MindFusion.Scheduling.Silverlight.MonthCellPresenter.cbeec0a808111aad1dd337d8d0a
35d972(c344a63b1287e10df4eee7c38a9892791 c4df335d6ed4bb0b313af384e60306a1c, Object cc1777c2113ae99e09937fa474900c19f)
bij A.c00a0a20e7f2f172e54529cdba3d483ba.Update(c344a63b1287e10df4eee7c38a9892791 type, Object parameter)
bij MindFusion.Scheduling.Silverlight.Calendar.cbeec0a808111aad1dd337d8d0a35d972(c34
4a63b1287e10df4eee7c38a9892791 c4df335d6ed4bb0b313af384e60306a1c, Object cc1777c2113ae99e09937fa474900c19f)
bij MindFusion.Scheduling.Silverlight.Calendar.OnItemRecurrenceReset(Object sender, ItemEventArgs e)
bij System.EventHandler`1.Invoke(Object sender, TEventArgs e)
bij MindFusion.Scheduling.ItemCollection.cac7722afe7a45b1ac5c756be5d334b1b(ItemEvent
Args ce5351bab187b73ac3548a065b6b6ff6d)
bij MindFusion.Scheduling.ItemCollection.cac7722afe7a45b1ac5c756be5d334b1b(Object cd89110e0d8f938c1c45fa78e7d96162c, EventArgs ce5351bab187b73ac3548a065b6b6ff6d)
bij MindFusion.Scheduling.Item.c223fe7df29e0f6584284eb0894e76e5f(EventArgs ce5351bab187b73ac3548a065b6b6ff6d)
bij MindFusion.Scheduling.Item.set_Recurrence(Recurrence value)
bij MindFusion.Scheduling.Item.set_Recurrence(Recurrence value)
bij Trainers.Dialogs.EditTraining.<>c__DisplayClass2.<.ctor>b__0(Object s2, EventArgs e2)
bij System.Windows.Controls.ChildWindow.OnClosed(EventArgs e)
bij System.Windows.Controls.ChildWindow.Close()
bij System.Windows.Controls.ChildWindow.set_DialogResult(Nullable`1 value)
bij Trainers.Dialogs.EditTraining.OKButton_Click(Object sender, RoutedEventArgs e)
bij System.Windows.Controls.Primitives.ButtonBase.OnClick()
bij System.Windows.Controls.Button.OnClick()
bij System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEve
ntArgs e)
bij System.Windows.Controls.Control.OnMouseLeftButtonUp(Control ctrl, EventArgs e)
bij MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, Int32 actualArgsTypeIndex, String eventName, UInt32 flags)

Thanks, both problems seem to be fixed.
1 thing I still notice is that the arrow sometimes appears in the calender at different locations while there are no events for that day.

The arrows keeps appearing also with this latest version. The reason seems to be that it appears correctly in a certain month because not all items are visible for that day. But when navigating to a previous/next month it remains visible at the same day of the month even if all items are visible for that day.