This seems to be a common problem. I had this happen on my old Dahon Mu XL. By the time I noticed it, the crack had progressed almost all the way around the circumference of the tube. Luckily, it didn't seem to be going anywhere else, so I kept using the bike. Eventually, the top of the seat tube disconnected. The bike would ride normally, but lifting it by the saddle would pull the seatpost out. I made a couple of aluminum "stays", connecting the seatpost latch to the frame's rack mounts. The frame lasted another 6,000 miles as my "winter" bike.

My next bike, a Tern Verge S11i, developed these same cracks. I was looking for them, so I caught them early. Rather than drill "stop holes", I greatly enlarged the stress-relief at the bottom of the seat tube slot. Several thousands of miles later, the cracks have not returned.


Cracks are seen radiating out from the seat tube. The bike had about 4,000 miles on it.



The modified stress-breaker. The radius of curvature is now large enough to resist the recurrence of cracks after several thousand miles. The defect on the left is the result of a slip with the Dremel tool.



The disconnected seat tube top and latch are seen here. The homemade "stays" allow the bike to be picked up by the saddle.