Usually bolts are put in groups with more than one for redundancy. Special heads could be a sign of older bolts, including custom one-offs, or older style bolts. Many older bolts are not to be trusted, including the old star dryvins. Without pictures though, it's hard to speculate.
If you want to read about how to recognize and replace bad bolts, there's good material out there on the interwebs and you could also take a class at a good, reputable climbing/mountaineering training place, here's one site to get you started: