Ref: Blockchain Based Smart Contracts : A Systematic Mapping Study (Maher Alharby and Aad van Moorsel)

Smart contract issues

Codifying issues

• Difficulty of writing correct smart contracts
• Inability to modify or terminate smart contracts 
• Lack of support to identify under-optimised smart contracts
• Complexity of programming languages

Security issues

• Transaction-ordering dependency vulnerability
• Timestamp dependency vulnerability
• Mishandled exception vulnerability
• Re-entrancy vulnerability
• Criminal smart contract activities
• Lack of trustworthy data feeds ‘Oracles’

Privacy issues

• Lack of transactional privacy
• Use of ‘Hawk’ tool 
• Lack of data feeds privacy

Performance issues

Sequential execution of smart contracts