The bitcoin blockchain system itself is pretty safe against hacking, as it does not rely on any central authority to keep data safe from manipulation or spying. All transactions are visible, and the record of valid transactions is secured by the block confirmation system (mining). There are attack routes (most famous is the "50 percent" attack) but most of them are academical and do not pose a real risk.
I don't have a complete overview of bitcoin history, but I think that the "hard fork" of 2013 was probably the most critical event in that it caused transactions to be lost after they were thought to be confirmed.

However, there have always been cases where human mistakes or badly written software have caused loss (or theft) of bitcoins:

• Users might forget their password, accidentally format their hard disk etc.
• Wallet software on PCs might be vulnerable to trojan attack targeted at bitcoin users, revealing user's private kease to an attacker.
• Badly written wallet software might use not-so-random random number generators when creating private keys.
• Exchanges may be vulnerable, allowing attackers to manipulate their account balance within the exchange and then withdrawing bitcoins.
• Any business handling bitcoins might be hit by inside jobs, which can also involve hacking of computer systems to keep the theft undetected for some time.

Onkel Paul