I often carry the Yojimbo IWB and I have never experienced a problem. Obviously, you just have to make sure that the pivot is not too loose.
Initially, the pivot of my Yojimbo was rather loose and the blade would open with little force. This was good for fast/effortless openings but I wasn't confident about keeping it closed in my pocket. I simply tightened the pivot slightly and now it feels very secure in the closed position. Find the right pivot tightness and you shouldn't have any inadvertent openings.
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