From http://blogs.msdn.com/ianjo/archive/2006/01/31/521078.aspx:
“ “Halloween protection" in database systems refers to a solution for a problem that can occur in update queries. The problem occurs when an update itself affects the rows selected for update. For example, imagine a company wants to give every employee a 10% pay increase. If the update query is unlucky enough to walk the salary index then the same row may be selected, updated, the update moves the row ahead in the index, and then the row will be redundantly selected again and again. This problem is corrected by isolating the rows chosen from the effects of the update itself. For example, a SPOOL operation which stores all the rows to be updated outside of the context and any index can provide the necessary isolation. SORTs are also sufficient for isolation purposes.“