Consider the following code snippet:

IEnumerable<string> names = GetNames();
foreach (var name in names)
  Console.WriteLine("Found " + name);
var allNames = new StringBuilder();
foreach (var name in names)
  allNames.Append(name + " ");

Assuming that GetNames() returns an IEnumerable<string>, we are, effectively, doing extra work by enumerating this collection twice in the two foreach statements. If GetNames() results in a database query, you end up doing that query twice, while both times getting the same data.

This kind of problem can be easily fixed – simply force the enumeration at the point of variable initialization by converting the sequence to an array or a list, e.g.:

IEnumerable<string> names = GetNames().ToList();

The rest of your code can stay the same, because both array and list types implement the IEnumerable interface.