During the measurement of the Gate River Run a few weeks ago Toni Youngman thought her counter had malfunctioned as it was incrementing sporadically or not at all. Turned out the wire holding the tang to her spokes had broken, and fortunately was easily fixed.
A week later I was riding a course on a different bike than my normal bike, which was in the shop for a much overdue tuneup, when I noticed the same thing. Turned out the counter was pushed out from the hub just enough that the spokes began to bend the tang to the point it was no longer catching. I should have noticed this when I installed it on the wheel and used the extension. Fortunately there was an electrician working a few hundred yards away who supplied me with some thin wire that I could MacGyver around the spokes and tang. Lesson: always check the clearance between your hub and counter, and carry some wire with you for emergency repairs.