Submitted by 0x1337, also found by broccolirob
https://github.com/code-423n4/2022-05-rubicon/blob/8c312a63a91193c6a192a9aab44ff980fbfd7741/contracts/RubiconMarket.sol#L448-L449
https://github.com/code-423n4/2022-05-rubicon/blob/8c312a63a91193c6a192a9aab44ff980fbfd7741/contracts/RubiconMarket.sol#L525-L535
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. 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.
Refer to the bottom part of this article: https://docs.openzeppelin.com/upgrades-plugins/1.x/writing-upgradeable
As an example, the ExpiringMarket contract inherits SimpleMarket, and the SimpleMarket contract does not contain any storage gap. If in a future upgrade, an additional variable is added to the SimpleMarket contract, that new variable will overwrite the storage slot of the stopped variable in the ExpiringMarket contract, causing unintended consequences.
https://github.com/code-423n4/2022-05-rubicon/blob/8c312a63a91193c6a192a9aab44ff980fbfd7741/contracts/RubiconMarket.sol#L448-L449
Similarly, the ExpiringMarket does not contain any storage gap either, and the RubiconMarket contract inherits ExpiringMarket. If a new variable is added to the ExpiringMarket contract in an upgrade, that variable will overwrite the buyEnabled variable in ExpiringMarket contract.
Recommend adding appropriate storage gap at the end of upgradeable contracts such as the below. Please reference OpenZeppelin upgradeable contract templates.
bghughes (Rubicon) confirmed 
HickupHH3 (judge) commented:
bghughes (Rubicon) commented:
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bghughes (Rubicon) commented:
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