When a COM error occurs, .NET tries to map it to an exception in the .NET Framework and throws that exception. If the COM error does not map to any exception in the .NET Framework, this exception is thrown instead. It's the "couldn't find an exception" exception.

public class COMException : ExternalException {
// Public Constructors
   public COMException( );
   public COMException(string message);
   public COMException(string message, Exception inner);
   public COMException(string message, int errorCode);
// Protected Constructors
   protected COMException(System.Runtime.Serialization.SerializationInfo info,
       System.Runtime.Serialization.StreamingContext context);
// Public Instance Methods
   public override string ToString( );    
// overrides Exception
}

Hierarchy

System.Object System.Exception(System.Runtime.Serialization.ISerializable) System.SystemException ExternalException COMException