If this can be done to one crypto currency, one has to be sure it or something similar won't happen to others.

Almost $500,000 in Ethereum Classic coin stolen by forking its blockchain
Rollback attack let attackers spend 88,500 previously spent coins.

Attackers have stolen almost $500,000 worth of the Ethereum Classic digital currency by carrying out a compute-intensive hack that rewrote its blockchain, officials with Coinbase, one of the leading crypto currency exchanges, said on Monday.

The heist was the result of carrying out what's known as a rollback attack, which allowed the attackers to reorganize the Ethereum Classic blockchain, Coinbase security engineer Mark Nesbitt said in a blog post. From there, the attackers were able to "double spend" about 88,500 ETC, meaning they were able to recover previously spent coins and transfer them to a new entity. As a result, the coins were effectively transferred from the rightful recipients to new entities chosen by the attackers.

"We observed repeated deep reorganizations of the Ethereum Classic blockchain, most of which contained double spends," Nesbitt wrote. "The total value of the double spends that we have observed thus far is 88,500 ETC (~$460,000)."

Stated a different way, a rollback attack generates a new fork of the blockchain. This causes nodes to replace the original blockchain with the new one and makes it possible for attackers to reverse previously made transactions. Rollback attacks require control of a substantial fraction of the total hashpower devoted to generating the coin's blockchain for a period long enough to pull off the attack. Bitcoin creator Satoshi Nakamoto warned of the key limitation in his white paper introducing the digital coin.

Coinbase paused movements of affected ETC funds to prevent any double spends from hitting its users. Meanwhile, the Kraken Exchange temporarily halted ETC deposits and withdrawals and plans to bring ETC funding back online once exchange officials believe it is safe to do so. ETC officials, for their part, have confirmed that double spends are affecting the currency, but they have yet to say more.

http://arstechnica.com/information-technology/2019/01/almost-500000-in-ethereum-coin-stolen-by-forking-its-blockchain/

It'll hopefully only hit the greedy !

The banks are so off the nose that people are looking at alternatives.

I personally would not trust something I can't hold in my hand.