Most recent was not technically flat (yet), but left alone would have been. A very fine wire - Looked like something from a wire brush, maybe even finer.
Before that, it was a 1.5 inch roofing nail. The point caught just enough in a small tread slot (between the rolling face of the tire and the sidewall) to go in, puncture into the tube and out, and then back out of the tire through the sidewall before hitting the rim. For a few seconds I wondered what that "clank clank" was when I applied the rear brake. It was the head of the nail hitting the canti brake arm. The tire had only about 350 miles on it, but it was history. Two holes in the sidewall, with matching tube holes.
Just to minimize my chances of getting more flats, I'll also mention the embedded glass micro-"pyramid" in my front tire that took two tubes before I excavated it, as well as the four (count 'em,
four) cactus thorn holes my daughter and I had between two tires November before last. Also, there was a furniture tack that laughed at the Slime in an old 26" tube (previous bike).
All but the first two were before April, but when it comes to flats and superstition, one can never be too careful...
