I've had to throw away two tanks so far. One got a little misshapen from biting down on it when I needed my hands for a few minutes (held it in my teeth too tightly I guess) and the other just quit seating properly this morning for no apparent reason (no idea what I did... maybe I sat on it in the car or something... beats me).
I bought a bunch of tanks when I got my ego-t a few weeks ago. I immediately filled 10 tanks. 3 with my favorite flavor, 2 with my second favorite, and the rest with assorted other flavors. I used a ball point to mark the rubber caps with what was in the tanks and I put the whole bunch in my carry kit. - The tanks I had to throw away were probably only filled three or four times each, but a couple with my favorite flavor have been filled too many times to count with no issues.