It looks like it was a bug in Bitcoin software ... see message from slush pool "21.05.2013
Bug in bitcoind caused many invalid blocks generated in recent hours. Thanks to cooperation with bitcoin developers, a bugfix has been deployed. Pool is now back in normal operation."

You just burned electric power for nothing ...

This is also the problem of the Pool managers (generally young people ...) They roll rapidly new software build and forget the good old sentence (if it's not broken, don't fix it)

the old programmer.