it is likely that you are talking about a hole in the atrio-ventricular septum. it's not all that rare, and i do know a few people who have had it. one of my friends in primary school had it and had to have a surgury to mend the problem. before the operation, she was often feeling dizzy / headaches and would feel very weak for no particular reasons sometimes. otherwise she was fine, and after the operation (which was about 7 years ago), she's been leading a completely normal life and there isn't much of an issue. the success rate of such an operation is quite high these days so if you want to have it, there's not much need to really worry about it.