With the Global Interpreter Lock (GIL) removed in Python 3.14 (in the no-GIL build), true multi-threading is now possible, which opens the door to data races—where multiple threads access and modify shared data concurrently, leading to unpredictable behavior. Previously, the GIL implicitly prevented many race conditions. Now, developers must handle concurrency explicitly using thread synchronization mechanisms like threading.Lock, RLock, Semaphore, or higher-level tools like queue.Queue. Careful design, such as using immutable data structures, thread-safe collections, or switching to multiprocessing or async paradigms where appropriate, is essential to avoid bugs and ensure thread safety.