The testing transactions were all "Pay 0.0001XLM", and there is far enough XLM to send. 5000 channel accounts were set up to try to prevent bad seq, but unfortunately it still happens as TPS > 100. Js-stellar-sdk was used.
On higher TPS testings, I encountered two similar bad sequence error codes.
To my understanding, fee is charged once a tx is submitted to the network for consensus. However, as I tested with Laboratory, if I am NOT using the upcoming seq_num (used or ) for this simple payment, it does not charge the fee... Any idea why the former case happened?