This is the kind of thing that would drive me nuts until the solution was found, and would continue to bother me afterwards:
Customer called, "random letters appear on screen." I went onsite.
Found they recently decommissioned a wireless keyboard by tossing it in the closet but not removing batteries or dongle.
Every time someone rifled through the closet, random letters would appear.
— Jonathan Johnson (@LaughingBrook) November 22, 2018