This is a new one to me and iIve been builing cars over 30 year. Part of that time was for a mass volume "production" machine shop. Normaly unless the bolts are damaged or it is a very high performance application re-using head bolts is acceptable. Did you install the heads yourself or have them done and are you sure that they were torqued to spec? Were the bolts properly LIGHTLY oiled for installation or installed dry? Dry installation can give a false torque reading that could possibly cause this. Lastly-what type torque wrench did you use, and if it is not yours where did you obtain it? "Clicker" type wrenches are easily knocked out of calibration if dropped or abused. If they are turned down below the minimum measured setting it will throw them off and possibly damage them. I am sure rental/loan-a-tool places seldom if ever check calibration. By international standard these wrenches are allowed +/- 4% to 6% at calibration. If a "Beam" style wrench or Digital wrench was used there should be no problem. Just trying to help GOOD LUCK