Common Mistakes to Avoid When Writing Smart Contracts

Smart contracts are self-executing agreements with the terms of the agreement directly written into code. They run on blockchain networks, ensuring transparency, security, and automation. However, writing smart contracts requires precision, as even minor errors can lead to significant financial losses or security vulnerabilities. In this article, we will explore the most common mistakes developers make when writing smart contracts and how to avoid them.

1. Not Following Best Practices in Security

Overlooking Reentrancy Attacks

One of the most critical security vulnerabilities in smart contracts is reentrancy attacks. These occur when an external contract repeatedly calls back into the original contract before the initial function execution is complete. The infamous DAO hack in 2016 was due to a reentrancy vulnerability, resulting in the loss of millions of dollars.

To prevent reentrancy attacks, developers should use the Checks-Effects-Interactions pattern. This means validating conditions first, updating the contract’s state, and only then interacting with external contracts. Additionally, using reentrancy guards (such as OpenZeppelin’s ReentrancyGuard) can help block recursive calls.

Ignoring Integer Overflows and Underflows

Another common security issue is integer overflow and underflow. Since Solidity versions before 0.8.0 do not automatically check for these, arithmetic operations can lead to unexpected behavior. For example, if a uint8 variable reaches 255 and increments by 1, it will wrap around to 0 instead of throwing an error.

Developers should use SafeMath libraries (or rely on Solidity 0.8.0 and above, which includes built-in overflow checks) to prevent these issues. Failing to do so can allow attackers to manipulate contract balances or bypass critical conditions.

2. Poor Error Handling and Input Validation

Not Using Require and Revert Statements Properly

Smart contracts must validate inputs and conditions rigorously. Failing to use require(), assert(), and revert() statements correctly can leave contracts vulnerable to exploitation. For instance, if a function transfers tokens without verifying if the sender has sufficient balance, an attacker could drain funds.

Developers should implement strict checks before executing critical operations. For example:

function transfer(address recipient, uint256 amount) public {
require(balanceOf[msg.sender] >= amount, “Insufficient balance”);
balanceOf[msg.sender] -= amount;
balanceOf[recipient] += amount;
}

This ensures the function reverts if the sender lacks sufficient funds.

Allowing Unrestricted Access to Critical Functions

Many smart contracts have administrative functions that should only be callable by the owner or authorized parties. If access control is not enforced, malicious actors could modify contract behavior, withdraw funds, or pause operations.

Using OpenZeppelin’s Ownable or AccessControl contracts helps restrict sensitive functions. For example:

function withdrawFunds() public onlyOwner {
payable(owner).transfer(address(this).balance);
}

The onlyOwner modifier ensures only the contract owner can execute this function.

Explore more: Top 5 Real-World Examples of Smart Contracts in Action

3. Gas Inefficiencies and Costly Operations

Using Loops Unnecessarily

Loops in smart contracts can be dangerous, especially when iterating over dynamic arrays with unknown sizes. If an array grows too large, a transaction may run out of gas and fail, wasting resources.

Developers should avoid unbounded loops and instead use mappings or pagination to handle large datasets. For example, instead of looping through all users to distribute rewards, consider tracking rewards individually or processing them in batches.

Not Optimizing Storage Usage

Storage operations in Ethereum are expensive. Writing and reading from storage consumes significant gas, so minimizing storage usage can reduce transaction costs.

Developers should:

• Use memory instead of storage for temporary variables.

• Pack smaller variables into a single storage slot (e.g., using uint128 instead of uint256 where possible).

• Avoid redundant storage updates.

4. Lack of Proper Testing and Auditing

Skipping Unit Tests and Testnets

Many developers rush to deploy contracts without thorough testing, leading to undetected bugs. Writing comprehensive unit tests using frameworks like Hardhat, Truffle, or Foundry is essential.

Additionally, deploying contracts on testnets (such as Ropsten or Goerli) before mainnet launch helps identify issues in a real-world environment without risking real funds.

Ignoring Third-Party Audits

Even experienced developers can overlook vulnerabilities. Professional smart contract audits by security firms (like Quantstamp, CertiK, or ConsenSys Diligence) help identify risks before deployment.

Auditors review code for logic flaws, security vulnerabilities, and gas optimizations, reducing the chances of exploits.

5. Poor Upgradeability and Maintenance

Using Immutable Contracts Without Upgrade Mechanisms

Once deployed, traditional smart contracts are immutable. If a bug is discovered, fixing it without an upgrade mechanism can be impossible.

Developers should consider using proxy patterns (like Transparent Proxy or UUPS) to enable upgrades while preserving the contract’s state. OpenZeppelin’s Upgradeable contracts provide secure templates for this purpose.

Not Including Emergency Stop Functions

In case of a critical vulnerability, contracts should have a failsafe mechanism to pause operations. An emergency stop (or “circuit breaker”) allows the owner to halt suspicious activities temporarily.

bool public paused;

modifier whenNotPaused() {
require(!paused, “Contract is paused”);
_;
}

function pause() public onlyOwner {
paused = true;
}

This prevents further interactions until the issue is resolved.

Conclusion

Writing secure and efficient smart contracts requires attention to detail, adherence to best practices, and rigorous testing. By avoiding common mistakes such as reentrancy vulnerabilities, poor error handling, gas inefficiencies, and lack of audits, developers can build robust and reliable decentralized applications.

Always stay updated with the latest security trends, use battle-tested libraries, and prioritize thorough audits before deploying contracts to the blockchain. Smart contract development is a high-stakes field—mistakes can be costly, but proper precautions can ensure long-term success.