Old client loses inputs regularly. Balance is inflated falsely.
The balance doesn't get inflated though as the orphans don't count in the balance. An orphan is a found block that gets rejected by the network because someone else found the block a bit faster than you and they get the reward which will be added to their balance not yours.
The balance bug was never seen before new client had been released. Visible balance increases from time to time in old client by input size (~600 coins).
It is unclear whether staking power rises too really or not. I tried zapwallettxes command before balance became inflated enough to notice difference in mined block amount per day.
Ah yes, I know what you're talking about now. I've seen that bug in the old client as well. It was an issue with the wallet though as on the blockchain the balance is always correct. Restarting the old wallet usually sets the balance correct again.
I haven't seen that bug in the new client.