Just a summary on Poloniex, so everybody can read it here.

Huntercoin balances are not affected. Bitcoin balances are, but will be reimbursed.
Apparently somebody found an error, which let him withdraw more than he had in his account. 12.3% of Bitcoins (not funds, just bitcoins) were stolen. Poloniex said they will deduct 12.3% of all Bitcoin balances for now - again, just Bitcoin -  and reimburs everybody once they have made enough funds from fees.
They said trading and withdrawals will be frozen for another 10 hours or so.

I think they handled it very well.