diff --git a/src/Orchard.Web/Web.config b/src/Orchard.Web/Web.config
index c337ae4ce..e89891961 100644
--- a/src/Orchard.Web/Web.config
+++ b/src/Orchard.Web/Web.config
@@ -63,8 +63,6 @@
         <add assembly="System.Web.Abstractions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL"/>
         <add assembly="System.Web.Routing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL"/>
         <add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
-        <remove assembly="mscorlib" />
-        <remove assembly="System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
         <remove assembly="System.Configuration, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
         <remove assembly="System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
         <remove assembly="System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
diff --git a/src/Orchard/Environment/Extensions/ExtensionLoaderCoordinator.cs b/src/Orchard/Environment/Extensions/ExtensionLoaderCoordinator.cs
index ad1c42f8b..298e9d5b3 100644
--- a/src/Orchard/Environment/Extensions/ExtensionLoaderCoordinator.cs
+++ b/src/Orchard/Environment/Extensions/ExtensionLoaderCoordinator.cs
@@ -247,7 +247,20 @@ namespace Orchard.Environment.Extensions {
         }
 
         private IEnumerable<ExtensionProbeEntry> SortExtensionProbeEntries(IEnumerable<ExtensionProbeEntry> entries, ConcurrentDictionary<string, DateTime> virtualPathModficationDates) {
-            return entries
+            // All "entries" are for the same extension ID, so we just need to filter/sort them by priority+ modification dates.
+            var groupByPriority = entries
+                .GroupBy(entry => entry.Priority)
+                .OrderByDescending(g => g.Key);
+
+            // Select highest priority group with at least one item
+            var firstNonEmptyGroup = groupByPriority.FirstOrDefault(g => g.Count() >= 1) ?? Enumerable.Empty<ExtensionProbeEntry>();
+
+            // No need for further sorting if only 1 item found
+            if (firstNonEmptyGroup.Count() <= 1)
+                return firstNonEmptyGroup;
+
+            // Sort by last modification date/loader order
+            return firstNonEmptyGroup
                 .OrderByDescending(probe => GetVirtualPathDepedenciesModificationTimeUtc(virtualPathModficationDates, probe))
                 .ThenBy(probe => probe.Loader.Order)
                 .ToList();
diff --git a/src/Orchard/Environment/Extensions/Loaders/CoreExtensionLoader.cs b/src/Orchard/Environment/Extensions/Loaders/CoreExtensionLoader.cs
index 9e6621ddb..f30b9b351 100644
--- a/src/Orchard/Environment/Extensions/Loaders/CoreExtensionLoader.cs
+++ b/src/Orchard/Environment/Extensions/Loaders/CoreExtensionLoader.cs
@@ -32,6 +32,7 @@ namespace Orchard.Environment.Extensions.Loaders {
                 return new ExtensionProbeEntry {
                     Descriptor = descriptor,
                     Loader = this,
+                    Priority = 100, // Higher priority because assemblies in ~/bin always take precedence
                     VirtualPath = "~/Core/" + descriptor.Id,
                     VirtualPathDependencies = Enumerable.Empty<string>(),
                 };
diff --git a/src/Orchard/Environment/Extensions/Loaders/IExtensionLoader.cs b/src/Orchard/Environment/Extensions/Loaders/IExtensionLoader.cs
index eb904148a..d3890d616 100644
--- a/src/Orchard/Environment/Extensions/Loaders/IExtensionLoader.cs
+++ b/src/Orchard/Environment/Extensions/Loaders/IExtensionLoader.cs
@@ -9,6 +9,7 @@ namespace Orchard.Environment.Extensions.Loaders {
     public class ExtensionProbeEntry {
         public ExtensionDescriptor Descriptor { get; set; }
         public IExtensionLoader Loader { get; set; }
+        public int Priority { get; set; }
         public string VirtualPath { get; set; }
         public IEnumerable<string> VirtualPathDependencies { get; set; }
     }
diff --git a/src/Orchard/Environment/Extensions/Loaders/ReferencedExtensionLoader.cs b/src/Orchard/Environment/Extensions/Loaders/ReferencedExtensionLoader.cs
index 9bee57fd7..c3d4581b1 100644
--- a/src/Orchard/Environment/Extensions/Loaders/ReferencedExtensionLoader.cs
+++ b/src/Orchard/Environment/Extensions/Loaders/ReferencedExtensionLoader.cs
@@ -1,4 +1,5 @@
-using System.IO;
+using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using Orchard.Environment.Extensions.Models;
 using Orchard.FileSystems.Dependencies;
@@ -59,6 +60,7 @@ namespace Orchard.Environment.Extensions.Loaders {
             return new ExtensionProbeEntry {
                 Descriptor = descriptor,
                 Loader = this,
+                Priority = 100, // Higher priority because assemblies in ~/bin always take precedence
                 VirtualPath = assemblyPath,
                 VirtualPathDependencies = new[] { assemblyPath },
             };
diff --git a/src/Tools/Orchard/HostContext/CommandHostContextProvider.cs b/src/Tools/Orchard/HostContext/CommandHostContextProvider.cs
index 082bc3c84..12505b9db 100644
--- a/src/Tools/Orchard/HostContext/CommandHostContextProvider.cs
+++ b/src/Tools/Orchard/HostContext/CommandHostContextProvider.cs
@@ -113,7 +113,12 @@ namespace Orchard.HostContext {
         }
 
         private static CommandHost CreateWorkerAppDomainWithHost(string virtualPath, string physicalPath, Type hostType) {
-            ClientBuildManager clientBuildManager = new ClientBuildManager(virtualPath, physicalPath);
+            var clientBuildManager = new ClientBuildManager(virtualPath, physicalPath);
+            // Fix for http://orchard.codeplex.com/workitem/17920
+            // By forcing the CBM to build App_Code, etc, we ensure that the ASP.NET BuildManager
+            // is in a state where it can safely (i.e. in a multi-threaded safe way) process
+            // multiple concurrent calls to "GetCompiledAssembly".
+            clientBuildManager.CompileApplicationDependencies();
             return (CommandHost)clientBuildManager.CreateObject(hostType, false);
         }
     }