The Random::DEFAULT constant isn't a alias for the Random::PCG32 implementation (sadly) but a global instance of it. It's initialized once then shared across all the threads, but the algorithm is thread unsafe (two or more threads generating a random number will mess with the internal state).

A solution could be to deprecate the constant, to introduce a thread local instance of the default RNG, and to expose it as Random.default.

EDIT: even with a thread local, the Enumerable#sample method is still unsafe for types whose #each may yield the current fiber because it takes then reuses a reference to the default instance (thread local) while the fiber can be resumed by any thread (oops).

EDIT: same for Crystal::System::File.mktemp since it tries to open on each iteration, which may yield the fiber.