Affected versions of this crate called mem::zeroed() to create values of a user-supplied type T. This is unsound e.g. if T is a reference type (which must be non-null). The flaw was corrected by avoiding the use of mem::zeroed(), using MaybeUninit instead.
Affected versions of this crate use orderings which are too weak to support this data structure. It is likely this has caused memory corruption in the wild: https://github.com/crossbeam-rs/crossbeam/issues/97#issuecomment-412785919.
Even if an element is popped from a queue, crossbeam would run its destructor inside the epoch-based garbage collector. This is a source of double frees. The flaw was corrected by wrapping elements inside queues in a ManuallyDrop.