- Edited
Having a scrambled PIN pad also indicates the possible existance of a Duress PIN,
which likely prevents the desired initiation of the task it was designed for.
IMHO a solution would be a prefix of say 2 or 3 or more digit's before the keypad becomes scrambled.
And at that point I guess it would be wise to monitor excessive (ab)use of the backspace key - and act on that.