I've snapped 2 paddle shafts over the past few years - first one was bought s/h, so couldn't judge it on how it had been treated, so I purchased a replacement shaft from the same brand, only to snap it about 12mths later (same location - lower hand grip) - this time I was certain that I hadn't damaged it in any way. Both were full carbon (stiff) shafts and a very reputable brand if that makes any difference.
Both times, I was paddling on to small (fat/choppy) waves in a strong cross-off wind, and at 6'2" 110kegs with all my weight on it, I don't blame the shaft at all. I'm not saying I'm strong - moreso that I was trying too hard to catch a wave using poor technique & weight distribution for a fat, sloppy beach break that really should have been left to pass.
Anyway - I epoxied/sandwiched a sleeve onto both sides of the second snapped shaft with a couple of pieces of the first broken shaft & tapered off the ends as it snapped at the lower hand grip - which gives me a good gauge of where my hand should be at. Not ideal (and many purists won't like it), but very effective and hasn't failed me since I did it, nor has the epoxy shown any signs of fracture across the join or sleeves (I'm still giving it my all!). I had intended on only keeping it as a spare, but at this stage it's doing nicely. I'll probably go with a KeNalu Xtuff for my next paddle.