If MtGox needs frehsly mined coins to keep their exchange alive, they are probably dead. When and at which price did they spend their client's bitcoins so that now they have to rely on mining new ones?
A client sets his mining address directly to their MtGox deposit address. Different client tries to withdraw the funds. MtGox grabs a couple output it owns without regard to its age and tries to send it to the withdrawing client. The transaction is delayed because some of these outputs are fresh.
A freshly mined coin is not valid for 120 generations. That's to avoid transactions being voided if the very end of the blockchain switches (if the chain is A->B->C, and you spend coins mined in B in C, but then the chain is overtaken by A->D->E->F, your transaction is invalid (coins genrated in B won't appear in the new chain))
If MtGox needs frehsly mined coins to keep their exchange alive, they are probably dead. When and at which price did they spend their client's bitcoins so that now they have to rely on mining new ones?