A local variable of a struct/array type is declared without an explicit memory/storage location (older Solidity) or as an unassigned storage reference. It aliases slot 0 and writing through it overwrites unrelated state — often the owner or a critical mapping.
// function f() public { Data storage d; d.x = 1; } // d -> slot 0 // slot 0 is `owner` -> d.x = 1 overwrites owner with 0x...01.
// always specify the data location explicitly MyStruct memory s = MyStruct(...); // memory, not a dangling storage ref
Largest recorded losses in the same failure class — related, not the same bug:
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