Throw same exception instance multiple times

It's bad practice for various already stated reasons, but it will fail especially hard in multithreaded code, because Exception class is (obviously) not thread safe, and it's not immutable.

Consider this code:

class Program {
    static readonly Exception _test = new Exception("test");

    static void Main(string[] args) {
        ThreadPool.SetMinThreads(10, 8);
        var random = new Random();
        int num1 = 0;
        int num2 = 0;
        var tasks = new List<Task>();
        for (int i = 0; i < 10; i++) {
            tasks.Add(Task.Run(() => {
                try {
                    if (random.Next(0, 2) == 0) {
                        Interlocked.Increment(ref num1);
                        Throw1();
                    }
                    else {
                        Interlocked.Increment(ref num2);
                        Throw2();
                    }
                }
                catch (Exception ex) {
                    Console.WriteLine(ex);
                }
            }));
        }

        Task.WaitAll(tasks.ToArray());
        Console.WriteLine("num1: " + num1);
        Console.WriteLine("num2: " + num2);
        Console.ReadKey();
    }

    static void Throw1() {
        throw _test;
    }

    static void Throw2() {
        throw _test;
    }
}

Here we have 2 methods, Throw1() and Throw2() which both throw the same instance of exception from private field. Then we run 10 threads which call either Throw1() or Throw2() randomly and print what has been thrown. Example output of such code is:

System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
System.Exception: test
   в ConsoleApp8.Program.Throw1() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 52
   в ConsoleApp8.Program.<>c__DisplayClass1_0.<Main>b__0() в H:\VSProjects\SoHelp\ConsoleApp8\Program.cs:строка 32
num1: 6
num2: 4

So while Throw1() has been called 6 times and Throw2() has been called 4 times - all 10 stack traces we printed refer to Throw1() method.

So just don't ever do that, because there is absolutely no reasons to.