Taryn Pratt points out that replicas of data can contain stale data:
Last week at Stack Overflow we had an internal hack-a-thon, or as we call it, a make-a-thon. I was on the bug-bashing team, which is the team that attempts to fix smallish bugs we haven’t gotten around to fixing, due to other time-constraints. I was tagged to investigate a bug about duplicate badges being awarded because it looked to possibly be an easy fix in SQL. At first glance it looked simple enough, but once I started digging in, I figured out very quickly it wouldn’t be.
It’s an interesting problem, but no solutions in the post. It’s a hard problem.