rayTest results in missing geometry

[Dragonlord]

Just stumbled across this one. Doing a ray-cast against a thin box shape results in hits reported randomly 50% of time (the ray position faintly jiggles like a fraction of mm per frame). As far as I can see btGjkConvexCast is involved with calcTimeOfImpact. How comes ray-casting is so instable? The convex sweep test in btCollisionWorld works flawless in the very same situation but using ray-test instead causes flickering results. Why not using a simple ray-box test if a ray goes against a box? Why using btGjkConvexCast when it's not stable?