I've had one buckthorn fail me, but that piece was an experiment with 15 thorn knots on the back, so I dont think that counts. I got it out to 23" before it went crack, a tension failure between two knots. However, even with the tension failure, it still wouldnt break, even at full draw and beyond, until I actually stepped on the failing area.
Actually, that's probably because the break couldn't travel due to being in between two knots. Just thought of that.