Summary

No storage gap can cause storage collusions.

Vulnerability Details

For upgradeable contracts, there must be storage gap to "allow developers to freely add new state variables in the future without compromising the storage compatibility with existing deployments" (quote OpenZeppelin).
Otherwise it may be very difficult to write new implementation code. Without storage gap, the variable in child contract might be overwritten by the upgraded base contract if new variables are added to the base contract. This could have unintended and very serious consequences to the child contracts, potentially causing loss of user fund or cause the contract to malfunction completely.

Refer to the bottom part of this article: https://docs.openzeppelin.com/upgrades-plugins/1.x/writing-upgradeable

Impact

As an example, Distribution is intended to act as the base contracts in the project. If the contract inheriting the base contract contains additional variable, then the base contract cannot be upgraded to include any additional variable, because it would overwrite the variable declared in its child contract. This greatly limits contract upgradeability.

Tools Used

Manual Review

Recommendations

Recommend adding appropriate storage gap at the end of upgradeable contracts such as the below. Please reference OpenZeppelin upgradeable contract templates.