What is a millisatoshi? I thought satoshi is the smallest unit? I encountered 'millisatoshi' (msat) while browsing Lightning Network..., see screenshoot below:



And subsequently I found it mentioned here: 

https://github.com/ElementsProject/lightning/blob/master/README.md#sending-and-receiving-payments

Obviously, I am curious... I thought satoshi is the smallest unit?
It's not a mistake, it's literally a thousandth of a satoshi. A satoshi is the smallest unit for bitcoin, but lightning can transact with even smaller units while channels are open. The amount is rounded down to the nearest satoshi when the channel is closed and broadcast to the blockchain to adhere to bitcoins limit.