Hey my man, Ive had that happen a multiple times out of about 40 mining transactions..  If you double click the ? on transaction list it explains it more.  Heres the info on it.  Generated coins must mature 210 blocks before they can be spent. When you generated this block, it was broadcast to the network to be added to the block chain. If it fails to get into the chain, its state will change to "not accepted" and it won't be spendable. This may occasionally happen if another node generates a block within a few seconds of yours.

No worries though, Ive had crazy mined coins already.  Good luck.