Several points ... first, the glass that'll be on either side of the joint has very high tensile strength that would prevent such a breakage, so not to worry.
Second, it's hard to judge the situation from my perspective. For example, tests that I've done (and others have done) have failed to break the joint and breaks occurred on either side of the joint instead. So why did yours break in the joint itself? Dunno ... the possibilities are many. Dust, sanded instead of planed scarfing (sanding seals wood cells, cutting leaves them open to wick epoxy into them). Was each surface of the joint precoated carefully, repeatedly (brush) until the whole joint remained wet looking after letting it sit for 10 minutes? This guarantees that the epoxy has wicked into the wood cells as far as it'll go. Was the joint then filled with thickened epoxy right away or after the epoxy that you used to saturate the joint has cured? Right away is best. How thick was the thickened epoxy? It needs to be soft enough to flow well inside the joint as it's pressed together, thereby acting as a gap-filling mix. If the epoxy is too thick (too dry) and combined with not enough saturation of the wood cells prior to assembly, then it's possible that the wood itself drew epoxy out of the joint, leaving a too-dry mixture behind - which will be weaker. Was the joint pressed together hard enough to squeeze all the epoxy out of some areas? And for full strength, the epoxy should be kept warm (room temperature) and allowed to cure for at least a week before testing. Hard to say. I prefer to use a plane to cut scarf joint (no sanding), vacuum the dust out of each face of the joint, saturate very well before gluing, use a soft silica/wood flour mix for the gluing, and I want to see squeeze-out around the whole joint, but leave it loose enough that you know you're not squeezing all the epoxy out. I avoid stressing the joint at all until it's been glassed.
To fix a busted joint, scrape and/or sand to remove all lumpy epoxy remains until the glue faces are smooth again, then plane off a little more to open up the wood cells, vacuum well, then saturate and glue as described. Don't stress the joint until well cured and handle it gently until you can get some glass on it. If no time to glass the whole thing, then just glass a 12" to 18" long strip of glass tape over the joint on each side and that'll reinforce it until you finish glassing later.
Hope that helps
Brian
