My aluminum 2006 Dahon Mu XL developed a crack that originated in the "stress breaker" hole at the bottom of the seat tube slot. Unfortunately, I did not detect it until it was almost all the way around the tube. Eventually the tube separated from the rest of the frame at the junction with the main frame tube. This didn't make the bike unrideable, but if I lifted the bike by the saddle, the seatpost would come out along with the clamp. I fixed this by making a couple of "stays" from the frame's rack attachment points to the seatpost clamp. The bike was my "winter" bike for several more years.
Also at some point the frame cracked at the junction between the main frame tube and the head tube. My brother-in-law heliarc welded it; this repair lasted the rest of the bike's life.
My next bike was a Tern Verge S11i. The same thing started at the seat tube slot stress-breaker. This time I noticed at an early point in the crack's evolution. I increased the radius of curvature of the hole and several years and a few thousands of miles later, there's no recurrence of the crack.
The separated seat tube is pretty obvious. Less so is the repair at the head tube. Note the stays holding the seatpost clamp to the bike.