It is my understanding that leaking of shares might happen when something is going on with the pool which causes your miners to switch over to your backup pool. This flip flop between pools causes a loss in shares as during the switch over a percentage are lost. In this case, via IRC, I read that Eligius was under a DDoS and WizKid057 was doing a fine job combating it. Needless to say though, you would be mining fine for a while, it would hiccup, your miner would switch to a backup, then after a minute or so switch back. My various rigs have stopped doing that since late last night, so I am assuming things were handled, it's been smooth sailing for many hours. (And we've had a nice little slice of luck!)