If the Mask property of the control is set to Mask="" the RadMaskedNumericInput should allow users to input minus sign before entering any other symbol. Currently users are not allowed to visualize only minus sign in the control in NoMask scenarios. Will be available in Q1 2016 Release.
If you execute cut (Ctrl+X) command without any selection in the RadMaskedInput control, you receive a square as first symbol.
The issue is reproducible only in Silverlight. MaskedInput is validated and validation error is shown in the validation tooltip. After removing the maskedInput from the Visual tree, it still remains in the memory. Available in LIB version: 2014.3.1124
Use MaskedTextInput in Silverlight and press (Ctrl + Shift + Z). This will produce unexpected symbol.