I've had this symptom only twice, both in the last week: I wake my MacbookPro11,4 (mid 2015) from sleep, and I can't type my password to log in because every input keystroke is immediately deleted. Effectively, it's acting as if the backspace key is pressed down and repeating, though it is not and I've never experienced a stuck backspace key in any other context on this computer with any keyboard. This has happened both with the computer alone (opening it up to wake it), and in clamshell mode connected to an external keyboard and monitor, where I am just waking it with the external keyboard or mouse. Any ideas? Is it possible this is a malware symptom of some kind? Nothing else weird is happening with this machine right now, but this particular symptom forces me to hard restart.