It's happened from Coinbase and Multibit. Here is the FAQ for the Multibit one:
https://multibit.org/en/help/v0.5/help_support_sentBitcoinFromMultibitButTheyAreStuck.html Here is Multibit reasons for a transactions not being broadcast:
There are various reasons why a peer does not relay a transaction. In a rough order of frequency they are:
There are lots of tiny inputs (under 5432 satoshis) in your wallet and the transaction fires the anti-spam rules.
The fee is too small and the peer chooses not to relay it.
The transaction is a double spend. Doubles spends aren't relayed through the network.
Some other reason - this is not a definitive list.
Mine fit into option 4: "Some other reason - this is not a definitive list"
So basically NO reason???