Why does one transaction fail with "Out of gas" while another tx succeed with the same gas limit?

Question

During a recent NFT mint (contract), approximately 50 out of 920 transactions failed with an "Out of gas" error.

We discovered users with failed transactions used the same settings as others with successful transactions: they all minted via the website, used Metamask, and did not touch the gas limit setting.

Here's an example of a failed transaction with the same gas limit of a successful transaction.

We're trying to determine how this happened, was the contract at fault, and how do we avoid this in the future?

Answer 1

From the transaction trace with Tenderly https://dashboard.tenderly.co/tx/mainnet/0x15b500b479a04486a1bd355ca6791c084f9fe48a55a6f2615753adf071ac6fcf

It just run out of gas while minting the last token.

To deter why one address did work and the other didn't you'll have to consider the state of the contract at the time. For some operations, like modifying storage, the cost depends on the previous state. So it is possible that it will work for one address an fails for other.