Using a HashMap in a multithreaded environment without proper synchronization can lead to various issues, including data corruption, unpredictable behavior, and performance problems. The primary concern is that HashMap is not thread-safe, meaning that concurrent modifications can lead to an inconsistent internal state. get() method goes into an infinite loop, this can happen because of the way HashMap handles its internal structure, especially during resizing operations. Using ConcurrentHashMap is generally the recommended approach, as it is specifically designed for concurrent access and provides much better performance in multithreaded environments compared to other synchronization techniques.