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.

Currently, performing consecutively paste operation does not work like in TextBox. 
In TextBox the strings are concatenated but in MaskedTextInput the caret is in the beggining and only the first paste is successful.

The XAML team has recently reviewed the status of this feature request and will not be addressing it in future.
Consecutive Paste operation works in No-Masked scenario (Set Mask=""). Changing the paste behavior in masked scenarios would be a breaking change for some of our clients. 

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.

In MaskedNumericInput when Mask="p2" you can not delete/replace the minus "-" sign when you select all and type a digit.

With no-mask, select all then typing a digit will remove the negative sign symbol from the text.

As a workaround, users can use Mask="", FormatString="p0" and maskedInput:MaskedInputExtensions.Maximum="0.99".

Second Workaround:

<telerik:RadMaskedNumericInput Mask="p2" Value="-0.25" x:Name="input" PreviewKeyDown="MaskedInput_PreviewKeyDown"/>

        private void MaskedInput_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            string keyString = e.Key.ToString();
            if (keyString.StartsWith("D") && keyString.Length == 2)
            {
                if (this.input.SelectionLength == "-AA %".Length)
                {
                    this.input.ClearCommand.Execute(null);
                }
            }
        }

Edit: Actually this is default behavior for NumericIinput with default Masks #9.2. Select All on negative value then pressing digit does not remove the negative sign.
This is the reason we will mark this bug as Declined. Please use some of the mentioned workarounds.
Please submit a support ticket via our ticketing system if this feature is critical for your business requirement.

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.

The minus key on a Mac keyboard, doesn't toggle the sign of the values in the MaskedNumeric/CurrencyInput controls. However, if an external usb keyboard is attached to the machine, it can be used to toggle the sign as expected.

Placeholder is set to " " and the SelectionOnFocus is set to CaretOnBeggining. When the Mask receives the focus, the caret is under the MouseClick position, not in beggining.
After load and clicking in the center of the MaskedInput , no value can be inserted.

The xaml team recently reviewed the status of this issue and won't be addressing it in future. 
Placeholder set to string.empty or ' ' should be considered not supported. Please consider using Mask="" (no-mask) instead.

Hitting dot or comma on the keyboard doesn't move the cursor after the decimal separator in the MaskedNumeric/CurrencyInput controls in Mac

When the user inputs digits in the RadMaskedNumericInput and then inputs some Spaces, the Value property is not changed although the UpdateValueEvent="PropertyChanged". It is only changed when the focus is lost.

Select text in the RadMaskedTextInput then Ctrl + X cuts the text but Ctrl + V does not paste it.

Steps to reproduce:
1) Type 12345. Select 345.
2) Ctr + X.
3) Ctrl + V won't do anything. (345 is not pasted) the result is - "_______________"). 
Expected result is: "345". This  should replace the whole text with the copied one. SelectionStart is not changed after paste.

Notes:  This bug is only for the scenario Cut + Paste in the MaskedTextInput with Mask. 
Pasting in No-Masked MaskedTextInput (Mask="") should work much like pasting in normal TextBox (in the same scenario the result should be "12345"). 

Numeric input with  Mask="#7" or CurrencyInput with       Mask="c7" .

 UpdateValueEvent="LostFocus"

Value =3000

If you remove the digit "3" with backspace / delete and then type digit 4,  You will receive value"4" and text "0004".

The expected result is Text="4000" and value =4000.

Workaround:  Use Mask="" (no-mask).
=============
Fixed in R3 2016.

Japanese input symbols are not correctly displayed. Empty symbols are shown on left of the japanese symbols.
Exception occurs when entering  n japanese symbols with mask = an. For example when Mask = "a4", 4 symbols entered in MaskedTextInput. This might lead to a crash.

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

We have a RadMaskedCurrencyInput control defined as:
<telerik:RadMaskedCurrencyInput Margin="20" Culture="se-SE" Placeholder=" "/>.
If we focus the control and press the LeftArrowKey, the cursor is placed on the last (far right) position in the control which is incorrect.
=====
Reason for deletion: You can consider Placeholder = " " as a risky setting that might lead to unpredicted parsing results. We highly encourage you to avoid using it. Instead you can use no-mask. In this scenario you can use:

 <telerik:RadMaskedCurrencyInput Margin="20" Culture="se-SE" Mask="" FormatString="c2"/> 

In a DateTimeMaskedInput with Mask=hh:mm tt, if the value is 10:00, the user can't enter 04:00.
=============
Reason for decline: The pattern "hh:mm tt" cannot accept time of type "00:00". You can test this with DateTime.ToString() method which will return "12:00" if you give him time (0hours, 0 minutes) and pattern "hh:mm tt". When typing 0 in the begging of the MaskedDateTimeInput , the control automatically converts 00:00 to 12:00 which is expected.

Workaround: You can use Mask="HH:mm tt" or Mask="t".

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.

The AcceptReturn=True doesn't work in a MaskedTextInput with Mask="l20" (l is the small letter of L ).
Controls behaves expected when the Mask is "a20".

User should be able to input 123.1.1.10 , currently only 123.1__.1__.10_ is achievable.

IpAddress demo will be added in our SDK with Q2 2014.

In Insert Mode digits are not inserted properly. For example, with Value 123456 - when caret is between 3 and 4 pressing digit overwrites the digit 4 but is shouldn't