There are several concurrency issues in the [ConditionalWeakTable](https://github.com/dotnet/coreclr/blob/master/src/mscorlib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs), these have been fixed in CoreRT's [ConditionWeakTable](https://github.com/dotnet/corert/blob/master/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs). The redesign in CoreRT should be integrated into CoreCLR.