Each transaction on the Stellar Network has a sequence number. Transactions follow a strict ordering rule when it comes to processing of transactions for each account.
For a transaction to be valid, the sequence number must be strictly 1 greater than the sequence number stored in the source account entry when the transaction is applied.
For example, if you are using LOBSTR Vault and LOBSTR wallet, the source account is the Stellar account used in LOBSTR wallet.
For accounts with multisig setup, there is a probability of multiple unsigned transactions existing with the same sequence number, waiting to be signed and submitted to the Stellar Network.
Such transactions do not exist on the Stellar Network until all signatures are provided.
Only one of those transactions can ultimately occur on the Stellar Network with the given sequence number.
Once one of those transactions is accepted and applied to the ledger, other transactions will be rendered invalid and will not be accepted by the Stellar Network.
An attempt to sign a transaction with an outdated sequence number in LOBSTR Vault will fail with the 'Sequence number does not match source account' message.
Such transactions will be marked as ‘Invalid’ at the ‘Transactions’ screen of the LOBSTR Vault app.
You will need to remove such transactions from your transaction list manually by choosing the 'Deny' option at the 'Transaction Details' screen.
An attempt to sign a transaction will also fail for the same reason if the sequence number is greater than the sequence number stored in the source account entry by 2 or more.
In this case, a transaction will be considered valid once the sequence number stored in the source account entry reaches the required value.