The notification will look similar to the following:

"Your transaction x1xxx1x111xxx111xxxxxx11xxxxxx11xx1111xx sent from account Wallet xxxxx has not been confirmed after several days and will be reverted within BitGo at (date and time). You may wish to attempt to re-send the transaction after this time. Note that any other sends that spent outputs of this transaction will also be reverted at this time."

When a new transaction is created, each node will store the transaction in its memory pool and relay it to its peers. Transactions leave the memory pool either when they get confirmed, or when they are dropped by timeout or mempool overflow. (Also see: In this case, the transaction was unconfirmed for almost two weeks and will therefore be dropped from our node's mempool. As each node operator can specify their own mempool behavior and nodes might have learned about the transaction at different times, our dropping the transaction doesn't mean that it will be forgotten on the network altogether— other copies may survive. As long as all the inputs of the transaction remain spendable, the transaction will remain valid indefinitely and may be included in the blockchain at a later time. As of now, the transaction was not confirmed, so it is not part of the blockchain yet.

To prevent the timed-out transaction from getting confirmed at a later time, you should spend one of the old transaction's inputs in a new transaction to make the old transaction invalid. If you reuse one of the inputs when creating the replacement of the old transaction, you can make sure that only one of the payments to your recipient can be confirmed on the network.