Quote from: theta on October 09, 2017, 06:55:18 PM

Yesterday I sent a transaction with a very low fee (1 satoshi per byte). It stayed for a few minutes in the memory pool and was showing as zero confirmations on both sending and receiving wallets. Then after an hour or so it had disappeared from both, as if it hadn't occurred. I assumed it had dropped from the memory pool as it was very low fee. Today I thought of entering it again but when I turned my computer on I saw (on the watch only version of the wallet) that the transaction had gone through (>100 confirmations).
Yet at some point last night I could have sent it again from the sending wallet.
What could have happened? Could it be that the transaction was still in the memory pool but one node decided to drop it and the wallet (Electrum 2.9.3) reported it as dropped, but then a miner included it in a block and so it was followed by others? And if so, if I had sent it again, presumably the second transaction would have been eventually rejected?

The mempool of one node is not the mempool of every node. People have noes compiled differently. The electrum nodes you connect to will all probably have default values. This means that the transaction couls still be in the mempool of another node with a larger amount of space avaliable for it and a different set of rules. This set of rules would then mean that the node keeps relaying the transaction and it can get included in a block.
Also, a second transaction would be accepted unless the inputs of that transaction used the same inputs of the other transaction, then it would be rejected by the network but may still be in the mempool - just never get a confirmation.