To clarify what happened, you apparently sent 1500 BTC while you were not connected to the network and/or had network related issues at the time of the send. Your client knew you sent and subtracted it from your balance but the transaction was sitting queued up (in your client) ready to get pushed onto the network once you connected to some other nodes.
Until you successfully connect, the rest of the Bitcoin network and especially MtGox (your destination) had no idea that you intended to send 1500 BTC. Once you connected the transaction got pushed out onto the network and got included in a block shortly after, specifically block
#131746. At this point the network knows about your transaction.
The important thing is to always have all blocks downloaded and at least a couple connections established in your client else any transactions you intend to send/receive will not appear to be successful.
I hope this helps.