Should I do anything if I am cited for something that wasn't in my paper? I’ve just read a paper in which a previous paper of mine has been cited.  The line in which the citation happens is something like:


  It has been shown that technique X is successful in this problem [citation for my paper].


However, in my paper, I never mention technique X.

Should I do anything?  The paper that cites mine is otherwise fine and really doesn’t need a reference for their use of technique X, since they spend a lot of time developing it anyway.
Since you never mention their technique, it is quite likely it's a "typo" (they accidentally cited a different paper than they meant, which, at least, in my field happens more than 0% of the time), or they just misunderstood your paper (or you misunderstood your paper).  I agree with @aeismail that there's nothing you need to do, but if you want to get to the bottom of this, you could contact the authors, saying something like:

I noticed you cited my paper in ... about X.  However, that paper didn't consider X.  Did you mean to refer to another paper?  If so, I'd be interested in knowing which paper.