For some giant critical flaw to be found in the code, rendering it useless. Or a well-executed 51% attack that destroys the network
My greatest fear as a bitcoin holder is when the value of bitcoin will dump for long time, because as a holder of bitcoin i always wish that the value of bitcoin will always increase so when i hold it for long time i can make or earn many money, and my othere greatest fear as a holder is when my wallet is hacked, that was so devastating if thats will happen for me.