Depending on the contestation period value and the network conditions,
`Close` and `Contest` transactions could become invalid and be
silently rejected by the cardano-node to which they have been
submitted. This can happen, for example, if:
* The network is congested, with lot of transactions waiting to be
  included in a block,
* The node's connectivity to the network drops and the transaction is
  not propagated to block producers fast enough.

The hydra-node itself does not currently handle this situation and
therefore each client application needs to put in place some retry
mechanism depending on the time it should "normally" take to have the
transaction.