When you set the IsCurrencySymbolVisible to false and Mask="1.x" you are not able to fill the first digit. 
For example if you want to insert 1.234 and you press 1 the control fills 0.001 and all other digits can be added at the last position only.

Having a Value of type 550.123, then selecting "55" and changing it to 7 produces 7.123 instead of 70.123.

Possible workaround is using no-mask (Mask="") and FormatString.

We have MaskedTextInput with InputBehavior set to Replace.

When we select part of the value or the whole value and then insert a symbol, only the first selected symbol is replaced but not the whole selected text.

If you enter the placeholder character in a MaskedInput, it will be processed as an empty position. And if the InsertBehavior is Input, the positions containing the placeholder chars will be overriden if the user keeps entering characters after the end of the allowed input is reached.

Use the new implict mechanism. Attach the Windows8 _Wpf project instead of Windows8 dll. Xaml parse exception occurs runtime - 
"Cannot set unknown member 'Telerik.Windows.Controls.MaskedInput.PreviewInputTextBox.IsReadOnly'."

Note: you can try the workaround added here: http://stackoverflow.com/questions/6850713/weird-xaml-parsing-error-when-trying-to-set-textbox-isreadonly

In PreviewKeyDown we change the Foreground of the MaskedInput when the Key is Return. Then we revert the color on other key press. Changing the focus returns the color set on Return but it shouldn't.

SelectAll then Shift + Del makes the need of twice (Ctrl + Z) for successful undo operation.

When two or more MaskInput controls are focused at the same time, the application freezes.

Workaround:

Instead of 

  txtMask1.Focus();
  txtMask2.Focus();

Use

  txtMask1.Focus();
                Dispatcher.BeginInvoke(new Action(() =>
                    {
                        txtMask2.Focus();
                    }), DispatcherPriority.Loaded);

Setting the Extnesions Minimum and Maximum values restricts the input of values that fall in the designated area but that start with a digit lesser than the minimum value. For example in the MaskedNumeric/CurrencyInput, if you set MaskedInputExtensions.Minimum="2" and MaskedInputExtensions.Maximum="20", you can't enter 12 unless the current value is 2 and the cursor is positioned before the 2 - only then you can insert the digit 1 before the digit 2 to create an input of 12.

When the Value of the RadMaskedInput (all RadMaskedInput controls) is changed in code behind, the ValueChanged event isn't fired.

When using implicit styles mechanism and you merge Telerik.Windows.Controls.Input an exception is thrown.

If the XAML file is set with BUILD ACTION PAGE the issue is not reproducible.

Another workaround is to reference dlls only, not xaml theme files.

Issue is reported in several msdn / stackoverflow threads and it appears to be an MS-TextBox one. 
http://stackoverflow.com/questions/6850713/weird-xaml-parsing-error-when-trying-to-set-textbox-isreadonly 

If there is a whitespace in the Mask of the control and you try to paste a value containing whitespace, the control will trim the user input.

Example:

Mask="(###) ###-####

User Input: 1234567890

The MaskedText should be: (123) 456-7890

If you copy this input and try to paste it into a control with the same Mask you will get (123) 45-6789 which is not correct.

Possible workarounds !!!

1. In a scenario when the user will enter only digits in the MaskedTextInput for a phone number is to restrict the input by setting the following mask.

<telerik:RadMaskedTextInput EmptyContent="Enter digits" Mask="(d3) d3-d4" />

2. The second workaround is to use non breaking space in Mask, this way there won't be a match between space by the user and space in the mask symbols.

The MaskedTextInput Value is bound to a property whose value is modified in its setter.
 In this case the Value of the control isn't updated to the modified value of the business property.
Also if Binding Converter changes the Value, it is not updated too.
Other maskedInput controls - Numeric, Currency work in such scenarios.

In MaskedTextInput the ClearButton will make the Value "" but it should make it NULL. All other InputControls have null value after pressing ClearButton.

No-masked TextInput works fine - the ClearButton makes the Value Null.

As a workaround, you can use the ClearCommand, bind it to Command from your ViewModel and the execute handler can set the bound Value to null.

Mask is "a-a". Value is set to "M-M".

The MaskedTextInput control must internally coerce the Value to "MM".
Instead , MaskedTextInput'Value is "M-" which is wrong.

Other example of the same issue:
Bound Value is bbaabbaabbdd, Mask is "aa-aa-aa-aa-aa-aa"
The result is - value becomes bbbbbbddaaaa.

Workaround:
Use custom Mask Token (check help article: http://docs.telerik.com/devtools/wpf/controls/radmaskedinput/how-to/howto-create-custom-token.html)

 public class CustomToken : ITokenValidationRule
    {
        public bool IsRequired
        {
            get { return false; }
        }

        public bool IsValid(char ch)
        {
            return ValidChars.Contains(ch.ToString());
        }

        public char Token
        {
            get { return '$'; }
        }

        public TokenTypes Type
        {
            get { return TokenTypes.AlphaNumeric; }
        }

        private string myValidChars = "0123456789qwertyuioplkjhgfdsazxcvbnm";
        public string ValidChars
        {
            get { return myValidChars; }
        }
    }

    TokenLocator.AddCustomValidationRule(new CustomToken());
            InitializeComponent();

   <telerik:RadMaskedTextInput Mask="$-$"  

The Mask property of the RadMaskedDateTimeInput cannot be set or changed via binding. Internally it is always set to "d".

The Value property of the TextInput is not correctly updated when using the On-Screen Touch keyboard when using no mask (Mask=""). We managed to reproduce the issue on Windows 8.1, 10. The behavior is different in both Windows OS. To get the Touch Keyboard you can check the following link.

Windows OS  8.1 - When you enter text and pressed backspace key- the value property is not updated the first time.

Windows OS 10 - When you enter text and pressed backspace key the value property is updated correctly. But if you HOLD THE BACKSPACE KEY the Value property ignored this behavior.