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))
