HashAlgorithm.ComputerHash isn't thread safe,

src/NzbDrone.Common/Crypto/HashConverter.cs

@@ -1,6 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
 using System.Security.Cryptography;
 using System.Text;
 
@@ -8,12 +6,15 @@ namespace NzbDrone.Common.Crypto
 {
     public static class HashConverter
     {
-        private static SHA1 HashAlgorithm = SHA1.Create();
+        private static readonly SHA1 Sha1 = SHA1.Create();
 
         public static int GetHashInt31(string target)
         {
-            var hash = HashAlgorithm.ComputeHash(Encoding.Default.GetBytes(target));
-
+            byte[] hash;
+            lock (Sha1)
+            {
+                hash = Sha1.ComputeHash(Encoding.Default.GetBytes(target));
+            }
             return BitConverter.ToInt32(hash, 0) & 0x7fffffff;
         }
     }