title: Collection was modified; enumeration operation may not execute - why?
body: <p>I'm enumerating over a collection that implements IList, and during the enumeration I am modifying the collection.  I get the error, "Collection was modified; enumeration operation may not execute."</p>
  
  <p>I want to know why this error occurs when modifying a item in the collection during iteration.  I've already converted my foreach loop to a for loop, but I want to know the 'details' on why this error occurs.</p>