My favorite non-thread-safe part of Java is this: SimpleDateFormat YMD = new SimpleDateFormat("yyyy-MM-dd"); If you call YMD.parse() from multiple threads

My favorite non-thread-safe part of Java is this: SimpleDateFormat YMD = new SimpleDateFormat("yyyy-MM-dd"); If you call YMD.parse() from multiple threads, it will sometimes return the wrong date. Cleaning up the database after that bug was loads of fun.