You have the following scenario:

The UpdateValueEvent property is set to "LostFocus". You have bound the Value property of the control to a property from your ViewModel. 


When this property is changed programmatically from negative to positive several times, the Value property of the control isn't updated correctly.

It stays negative when it must stay positive.


Workaround (if applicable):
You can change the UpdateValueEvent property to "PropertyChanged" 

Other Possible workaround (if the bound property is named "Sum")

private void radMaskedNumericInputSum_ValueChanging_1(object sender, Telerik.Windows.Controls.MaskedInput.RadMaskedInputValueChangingEventArgs e)
{
    if (this.Sum != (double)e.NewValue)
    {              
        e.Handled = true;
    }
}

In order to reproduce,  you need to focus the control when you run the application.

1.When you press the backspace key it removes the last character and after pressing it a second time the character will be replaced with the removed character.

2. When paste(Ctrl+V) a text in the control, the text is ignoring the current position of the caret and position at the beginning of the control.

Mask="" (no-mask) and FormatStyring =n3 or n4 , n5.

Value is 0.

Typing 0 then ".", then 0, then 0, then 5 produces 0.5000 but it should produce 0.0050.

Fix available in LIB Version 2016.1.404.

Available in LIB version: 2016.3.1010

RadGridView with CellEditTemplate with RadMaskedDateTimeInput.

Select a GridViewRow. Press ALT  + N Key (or other key with letter/digit).

Argument Exception occurs in MaskedDateTimeGridViewEditor.



The issue is reproducible in Q3 2015 SP (1104) version of Telerik UI for WPF.
The fix will be available in Q1 2016 Release.

Allow entering an input in (exponent) scientific notation form for the RadMaskedNumericInput control

Mask="Pddd" should disallow letters.

However, after select all, you can type letter in the position of the first "d"

Available in LIB Version: 2016.1.125.

Bug details:
OS: Windows 10.
VS: 2015 professional
Telerik version: 2015.2 SP1

Code sample:
<Window x:Class="RadMaskedInputBug.MainWindow"
                xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
                Title="MainWindow">
    <Grid>
        <telerik:RadTextTimeInput />
    </Grid>
</Window>


The designer crashes and shows only the exception:
InvalidOperationException: Can only base on a Style with target type that is base type 'RadMaskedTextInput'.

UPDATE: 
Telerik has submitted a bug report to Microsoft. You can check its progress here: https://connect.microsoft.com/VisualStudio/feedback/details/2005303/designer-error-can-only-base-on-a-style-with-target-type-that-is-base-type-customcontrol-when-a-style-is-basedon-a-style-of-an-abstract-class 
On 21/12/2015 Microsoft commented that a fix has been submitted and will be available in the next update.

Fix available in LIB Version 2017.3.1009.

Currently users are not allowed to completely remove or customize the Undo/Redo actions in the RadMaskedInput controls. Consider adding extensibility points such as commands or events.

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

Currently the MaskedInput controls cannot handle properly Chinese input symbols from Google Pinyin or MS IME.

MaskedInput's code relies on methods from TextBox which do not fire when the input is not from keyboard.

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.

Currently the MaskedInput controls cannot parse strings containing such full-width symbols.

Actually the .net framework also cannot directly parse such symbols.

Pasting text in TextInput with No-Mans and UpdateValueEvent=LostFocus is not successful.

This operation clears the whole Value.

 <telerik:RadMaskedTextInput Mask="" UpdateValueEvent="LostFocus"/>

Available in LIB version: 2014.3.1305

The 'hh:mm' Mask is not applied correctly in the DateTimeInput when the control is focused.

For example if the date is 2014-05-23, 22:30, and the Mask property of the control is set to 'hh:mm' the displayed text should be 10:30. Currently, this text is displayed only if the input is not focused, otherwise the text changes to 22:30

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="$-$"  

I think it would be great to have a IsMultilineAllowed property for this control, as AcceptsReturn property is only designed to ignore the return key entered by the user.

Here is my post in the forum:
http://www.telerik.com/forums/paste-multi-line-text-when-acceptsreturn-set-to-false
========
Reason for Decline: The desired request is custom logic that could be implemented by inheriting the RadMaskedInput control and overriding HandlePaste.

Numeric or Currency Input with Value = Null.

Clicking minus key (OemMinus or NumpadSubtract) places minus but Shift + OemPlus (or Numpad Add) does NOT remove the minus sign.

Will be available in Q1 2016 Release.

Part of the Value or the whole value is selected in no-masked NumericInput.

Pasting a copied text is not successful. For example 4 is the value and 4 is selected. Pasting "123" produces "1" but it should produce "123".