the problem is 0.7 and earlier reject valid blocks due to a bug in 0.7 and earlier.
The reason the fork happened is because the 0.8 clients
accepted blocks that the 0.7 clients rejected.
The blocks being rejected by 0.7 are:
1. Possible to generate with 0.7 as well as 0.8
2. Perfectly valid blocks according to standard.
3. Being rejected because of a bug in 0.7