I got a quote from someone (Autoglass i think) for a small 3-4mm chip and the guy was honest enough to say that the chance of cracking the windscreen during the repair was so high he wanted me to make sure a full replacement windscreen was covered on my insurance policy (without effecting my no claims) in case it happened. He wouldn't even start until I checked my policy, which at the time was a bit of a pain, but makes sense that he was covering himself... No lighter i can remember, but i think he was suggesting the pressure of the repair gun squirting in the resin into the crack is enough to cause a crack to propagate.