Transactions on Nervos L1 are atomic and deterministic, meaning that they always happen as expected, or not at all.
In the case that a transaction is made using the Neuron wallet address before it reaches full sync, the transaction may or may not succeed, but there is no way to know for sure until the node has reached 100% synchronization. Neuron must be fully synchronized before it can properly display balances and consistently generate transactions.
In the case of transaction failure, the worst-case scenario is that you must make a new transaction. On L1, either a transaction is successful, or it does not get recorded at all.
Furthermore, you can verify if a transaction was successful or not on the CKB Explorer.
If your Neuron wallet is still not fully synced after an extended period of time or transactions continue to fail, feel free to consult the #support channel in our Discord for further assistance. You can also view and create issues on GitHub: https://github.com/nervosnetwork/neuron/issues. If you decide to open a new issue on GitHub, we recommend adding your debug information and providing further details about your computer specs and operating system. Additionally, it is recommended that you clear your cache (located in settings), make sure that VPNs are off and computer speed is not compromised, and try restarting the application and your system.