Are There Limits to Software Estimation?
Charles Connell submitted this analysis on software estimation, a topic which keeps coming up because it affects so many many programmers. Read this post about J.P. Lewis's earlier piece as well, if you'd like more background information.
and then the next day the database changes
and the next the business rules that determine the queries
and then there's a data change that would be faster to change by hand 1000 times than build the regexp to do it for you.
and then there's data corruption because of a solar flare.