I have seen this a couple times now, and now my friend has hit it.
Action: Send a bitcoin transaction to an address from the client. Client says "Money Sent".
Result: Transaction never shows up on the network (as seen at http://bitcoincharts.com/bitcoin
I don't know why this is happening, and I don't know what to tell my friend (whom I'm trying to get interested in bitcoin). In my case, I was able to restore my wallet from a backup, and thereby get rid of the nonexistent transaction. That seems like a pretty crappy thing to tell someone who's just getting started -- not going to lead to trust of the system.
Anyone know what's happening?