There is a ball bearing that locks the safety in the off position. It has to be in proper adjustment or it will walk on.
If you read my post, you will see that I recommend Gunter Pfrommer for all such work. Incidentally, I just received my ICD back from him today that had the same problem.
The odds are slim that you are putting the safety back on with your thumb and you can check this easily. Even though you think there is sufficient pressure to keep the safety on, that isn't necessarily true. Yes, I've had this problem more than once and it never was pilot error. I shoot thousands of rounds annually through my ICD's and have plenty of experience with issues like this.
