I've only been SSSSed three times so far. Once was on CO on a company-purchased ticket. Oddly enough, I was travelling with a TV crew, all of whom were toting as much gear as they could as carryons, whilst all I had was one tiny overnight bag and a book. Out of the 8 of us, I got the SSSS.

The other two occasions were last-minute-ticket-syndrome. Once at MKE, when an AA flight I had boarded went mechanical and they put me on YX & AK to get home, and once at FLL - my husband and I switched our flights from AC to US at the last minute to avoid weather cancellations in YYZ and we both got the SSSS. I had brought back some mini-jams from Harbour Island (all <100ml and sealed in a Kippie, of course!) and the TSA agents had fun checking each jam to make sure it really was guava and not some kind of fruity tropical explosive.