.net - Under what circumstance System.Collections.ArrayList.Add throws IndexOutOfRangeException? -

We are experiencing a weird bug in the production environment, we can not debug and can not remove the entry code. I'm trying to understand it, but I'm confusing me after the stack trace.

  System.IndexOutRangeException: The index was outside the range of the array on the system. The selection should only be thrown to  NotSupportedException  on the collection.orlist.ed (object value) ...  

add method.

I do not know what is happening here, you?

This thread proves to be not secure list

In this case, the updated counting countdown becomes corrupted and later it will fail goes. Wrap () should be stopped around the lock statement.

  lock (updated fill) {updatedFills.Add (fill); }