Transaction Dropped & Replaced?

Dropped Transaction

A transaction that is previously broadcasted on the network (but yet to be included in a block) can get dropped from the connected nodes. Dropped transaction happens more frequently during times when the network is busy.

There are several reasons for this:

The Ethereum nodes (i.e., Geth/Parity) that we are connected to has dropped the Pending transaction from the transaction pool (most likely due to a lower gas price compared to other pending transactions or hitting other limits).
There is a maximum amount of Pending transactions an Ethereum node can hold before certain transactions are dropped from its pending pool (this depends on the various settings/limits of the individual clients connected to the network).
If a dropped transaction is not re-broadcasted, it will not be included in a block. If this happens, it would be as if the transaction had not taken place in the first place at all.
If a dropped transaction is successfully re-broadcasted either directly or indirectly (by a different node), then it will re-appear as a Pending transaction.
The transaction fee (gwei) provided for the transaction was too low. Consider replacing the transaction with a higher gas price
For further assistance on dropped transactions, please contact the sender/creator of the transaction (i.e., your exchange, wallet provider, etc.)

Dropped & Replaced Transaction

A transaction can get dropped and replaced when a newly created transaction with the same FROM account nonce is accepted and confirmed by the network. And because it has the same account nonce as the previous transaction, it replaces the previous txhash.

Common reasons a replacement transaction is broadcasted :

The earlier transaction had a low gas price which would take a very long time to confirm, so a second transaction with a higher gas price was created to replace it
The Ethereum Node that the wallet/service connected to was not fully synced, and an incorrect nonce was used
User-initiated to replace or cancel a pending transaction

A transaction can be {Dropped} or {Dropped & Replaced}. For additional assistance, please contact the sender/transaction creator directly for more information.