Collection modified error

We are getting occasional exception errors on different pages that are using ajax control toolkit and I am not sure how to track down the source of the error.  Below is an example exception thrown every so often.

An unhandled exception occurred:
Message: Collection was modified; enumeration operation may not execute.

 Stack Trace:
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Linq.Enumerable.All[TSource](IEnumerable`1 source, Func`2 predicate)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetScriptReferences(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetCombinedScriptContent(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetCombinedScriptContentHash(HttpContextBase context, String[] bundles)
   at AjaxControlToolkit.ToolkitScriptManager.OnLoad(EventArgs e)
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) Host = BX722T