On a ride on my first tubular-equipped racing bike in 1965, I heard a tapping noise coming from the front wheel. Got off to investigate, found a little white teardrop-shaped object sticking out of the front tire that looked like the kind of pushpin that was used on cork bulletin boards in those days.

Tried to pull it out. It wouldn't budge.

Finally realized that it was a tiny bit of the tire's latex tube that had popped out through a cut but hadn't punctured.