Collection was modified after the enumerator was instantiated

Hi,
I get this error sometimes and I don't know what it means.  My application, a web service is connecting to a database using a connection string.  The connection string is encrypted in the web.config file.
Usually everything goes fine and I notice that I get this error mostly when I try to call the web service multiple times in rapid succession (or concurrently).
What might cause this error?  Searching around I see people have this error with collections.  I'm not modifying the web.config at all.  As far as I know it's just a normal call to get AppSetting in the web.config.
 
Type : System.Configuration.ConfigurationErrorsException, System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Message : An error occurred loading a configuration file: Collection was modified after the enumerator was instantiated. (web.config line 59)
Source : System.Configuration
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