Swift's actor model can lead to unexpected reentrancy when methods contain `await` points. A suspended task allows another task to start executing, causing interleaving where state changes become visible out of order. This isn't parallel execution, but a scheduling optimization that prevents deadlocks by letting waiting tasks progress.

The provided `Counter` actor example demonstrates this clearly. Three sequential `count()` calls each spawn a task that increments a private `value` and then sleeps. All tasks reach the print statement showing `value = 3`, because each task reads the final state after the others have already updated it, not their intermediate states.

This behavior is a deliberate trade-off in Swift's concurrency system. Interleaving ensures resource efficiency and avoids system-wide stalls. Developers must design actor methods to be idempotent or manage state carefully, understanding that suspensions create potential re-entry points that can break naive assumptions about sequential execution.