The Doctrine of Equivalents can sometimes be an irritating aspect of patent law. Patent applicants tend to draft broad claims to begin with, and modern claim construction law likewise tends towards broad constructions. Once you add DOE on top, it can start to feel like the patent covers anything and everything.
But there is a boundary. Under the doctrine of "vitiation," courts have held that DOE can't be used to extend a claim to the oppose of what was claimed.
We got a good example of that this week from visiting judge Kennelly, who granted a motion for summary judgement of non-infringement after a seemingly fully-articulated DOE claim (DOE claims are often not-so-well articulated).