0.8 clients were wrong to accept it, because doing so caused a failure of consensus. Unfortunately the limit that was hit (which isn't quite as simple as 'size

0.8 clients were wrong to accept it, because doing so caused a failure of consensus. Unfortunately the limit that was hit (which isn't quite as simple as 'size') was implicit in BDB and how Bitcoin prior to 0.8 used BDB. So no one was aware of it. Extensive testing during 0.8 turned up several consensus breaking issues that were created during the database switch out, and very large blocks were tested... but the particular kind of very large block required to reveal this difference in behavior was not.