I am not sure I understand your question. The only case RadMaskedTextBox overwrite any previously entered character is when the maximum length of the input string has been reached and no additional character can be added to the value. In such situations, the textbox simply overwrites the next character. If this is what you meant, please note, that this behavior is dictated by the exact mask specified for input and is a requirement for preserving the validity of the new entered value.
the Telerik team