Per-thread initialization: MySQL internally counts the number of active threads, which causes a delay in my_end() if not all threads have exited. But there are threads that can be started either by Windows internally (often in TCP/IP scenarios) or by users. Those threads do not necessarily use libmysql.dll functionality but still contribute to the open-thread count. (One symptom is a five-second delay in times for PHP scripts to finish.)

Process-initialization: my_init() calls WSAStartup that itself loads DLLs and can lead to a deadlock in the Windows loader.