Like the oven cleaner, some things are very powerful and useful in their place, but you don't want to get any on you.
Contrast with Not Considered Harmful.
It originally came from 'Go To Statement Considered Harmful', by Ew Dijkstra, in Communications of the ACM, Vol. 11, No. 3, March 1968, pp. 147-148. You can read this classic paper online at www.acm.org 
. The phrase has been much misused over the years.
According to the following anecdote, however, the phrase was not due to Dijkstra anyway:
In 1968 the Communications of the ACM published a text of mine under the title "The goto statement considered harmful", which in later years would be most frequently referenced, regrettably, however, often by authors who had seen no more of it than its title, which became a cornerstone of my fame by becoming a template: we would see all sorts of articles under the title 'X considered harmful' for almost any X, including one titled "Dijkstra considered harmful". But what had happened? I had submitted a paper under the title 'A case against the goto statement', which in order to speed up its publication, the editor had changed into a 'Letter to the Editor', and in the process he had given it a new title of his own invention! The editor was Niklaus Wirth. -- Dijkstra (www.cs.utexas.edu , last paragraph.)
But this days there are many other things 'considered harmful': harmful.cat-v.org
Count 'em: www.google.com (442,000 on Nov 26, 2001) (625,000 on May 4, 2002)
...but you should be doing: www.google.com (a mere 49,000 on Dec 3, 2002, about 51,500 on Mar 21, 2003, and now 71,000 on Jan 6, 2004) -- An Aspirant
(247,000 on Feb 10, 2005) 1,620,000 20060517
See also:
For every feature of programming languages I have been able to find at least one publication claiming that it was harmful. -- Dick Botting
News flash! Dick Botting Considered Harmful!
Interesting; never saw an argument against the 'END' feature, sometimes called 'return'. j.b.nickson
If I understand it correctly, the Co Routine advocates would have something to say about that...'
There is a paper on the web, Considered Harmful essays Considered Harmful (www.meyerweb.com ), in which it is argued that most such essays tend to be full of dogmatic assertions, ad-hominen attacks, and specious arguments rather than well-reasoned positions (let alone mathematical proof or empirical evidence). The irony of the article is not lost on the author, fortunately.
Which raises the question: How do the set of "considered harmful" essays pertaining to computer science compare to CS papers in general? Even excluding the "considered harmful" essays; there is a lot of utter rubbish disguised as scholarship to be found within our field of study. Out of all the hard sciences and engineering disciplines, it seems that CS is the one which most closely resembles sociology when it comes to flamewars in the literature.
In medicine: Contra Indicated
In Soviet Russia: Harmful considers you!!!
See original on c2.com