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???