Bitcoin was designed to be decentralised, with multiple nodes on the network reaching a fair consensus, but mining pools have recentralised it, with a relatively small number of mining farm nodes creating most of the blocks, effectively controlling the network.
With each node having so much power it may be possible to attack or seriously disrupt the network, which could make people panic and cause a fall in price.
If one of the supernodes drops out (either permanently by choice, or semi permanently say due to a natural disaster etc) then a big chunk of the consensus protection also disappears, since the remaining nodes have even more power between them.
I don't know specifics about the mistaken transaction but it could be that the person created a raw transaction themselves, and forgot to send themselves the change. Your wallet balance consists of zero or more previous complete transactions, which need to be combined and/or split in order to create a new transaction. Any funds above the previous transaction combination you send to someone as a new transaction are given as a fee to the miner of the block which incorporates that transaction. It would be like splitting a $100 into twenties, attaching a note to one that it is intended for person X, then dropping all five twenties into a mail box, but without further specific instructions to return four of them to you. You intended to send $20 and get back $80 change, but you actually sent $20 and forfeited the $80 (the post office gets that
)