The designer of VS crashes with a "Key cannot be null" when custom markup extension is used in Style Setter. This reproduces in the sample app attached in the blog (http://www.telerik.com/blogs/bring-office-2016-inspired-style-to-your-app-with-ui-for-wpf) on the TextBlock of the RabRadioButton HeadingStyle (snapshot attached) if you open MainWindow in Designer. Runtime everything works as expected.

The RadGridView's is bound to an empty DataView and grouped by a GridViewComboBoxColumn. When adding items during runtime, setting the SelectedItem in code or selecting through the UI -> the SelectedItem remains null. 

The check mark of the RadioButton is misaligned when DPI settings are set to a higher than the default value, e.g. 125%.

The problem can easily be reproduced using the Telerik UI for WPF Demo application:

Cannot parse values when DisplayFormat is set to Long and AllowParsingWithoutSeparator is True.

" Collection was modified; enumeration operation may not execute.'" - The exception is observed when the CLR exceptions are enabled from Visual Studio. 

Edit:

The Ellapsed event handler of the Timer is executed in background thread but modifying chart data in a background thread is considered bad practice which is not supported by our component. This is the reason we are declining this bug report.

General solution is to schedule the data update to happen on the UI thread with Dispatcher. When in ViewModel, the following code can be used:

        private void OnTimerElapsed(object sender, ElapsedEventArgs e)
        {
            ViewModelBase.InvokeOnUIThread(() =>
            {
                for (int i = 0; i < Items.Count; i++)
                {
                    Items[i].X += (_random.NextDouble() - 0.5) * 0.1;
                    Items[i].Y += (_random.NextDouble() - 0.5) * 0.1;
                }
            });
        }

Tab selection does not work in the VS designer when switching between different views.

The panel allocates empty space for items that are hidden by setting their Visibility to Collapsed. Such items should be ignored when the panel arranges its items.

To work this around, instead of the Visibility property, you can remove and add the items in the ItemSource collection whenever you need to toggle their visibility.

If you define two UserControls in a project and you add two RadTabControls - one RadTabControl per UserControl, then in design-time you can only change the selection of one of the tab components.

Setting transparent background to RadBookItem doesn't work properly.

When an application is built with .net version 4.7, the paragraph dialog is clipped in the bottom.

 Group rows which are not present in the view are not visualized when the window is maximized.

A possible solution here is to set the GroupRenderMode property to Flat.

When there are items with longer text than the FilteringControl's default width, the FilteringControl resizes when scrolling to these items. This behavior is observed with all themes, different from Windows 8 and Windows 8 Touch.

"Win32Exception (0x80004005): Not enough storage is available to process this command" is thrown when multiple RadDocument instances are created in background threads. 

This is due to the fact that RadDocument contains MailMergeDataSource object, which is DependencyObject. When multiple dependency objects are created in different threads, they are not property freed by thread's Dispatcher.

Workaround 1: Microsoft provided workaround in a bug report here:
https://connect.microsoft.com/VisualStudio/feedback/details/620588/system-componentmodel-win32exception-0x80004005-not-enough-storage-is-available-to-process-this-command , namely:
------------------------------
Put the following code:

Dispatcher dispatcher = Dispatcher.CurrentDispatcher;
dispatcher.BeginInvokeShutdown(DispatcherPriority.Normal);
Dispatcher.Run();

anywhere in the background thread processing the RadDocument.
-----------------------------

Workaround 2: Use pooling of the used threads. The problem is that Task Parallel Library (which has built-in pooling) does not use STA threads by default, but this can be achieved by using custom TaskScheduler (attached, the code is get from here: https://code.msdn.microsoft.com/ParExtSamples ):

            var parallelOptions = new ParallelOptions()
            {
                TaskScheduler = new StaTaskScheduler(Environment.ProcessorCount)
            };

            Parallel.For(0, 10000, parallelOptions, cc =>
            {
                    var document = new DocxFormatProvider().Import(data);
                    byte[] pdfBytes = new PdfFormatProvider().Export(document);

            };

Exporting documents that contain PNG images SMask and ImagesDeflaterCompressionLevel is set to a non-default value may produce a document in which those images are not processed correctly by Adobe reader.

On export to HTML tab stops are converted to non-breaking spaces ( ). Sometimes the width of the all nbsp-s becomes different than the width of the tab stop. When document which contains grid-like data is exported to HTML its columns are not aligned.

The content of the cell is set from .LoadContent() method and is returned from the CreateCellElement method of the column. 

One possible scenario for a fix is to change that behavior and apply directly the template as WPF DataGrid does. However, this is a huge breaking change with CreateCellElement method is widely used. The second approach is to leave the code as it is right now, return the element from method of DataTemplate and set the ContentTemplate property of the cell to that DataTemplate. This will cause the content to be loaded twice. Which will degrade the  

So, as it turns out the solution is either to introduce breaking change or degrade the performance. Both of them are against our believes and efforts to improve the controls. 

Our suggestion is to use a workaround -  setting triggers directly to the style. We do hope you understand our concerns. 

The fix for this issue will be available with the next LIB (version 2018.3.1224) expected on Monday, December 24.