I’d like verifiable (either print or web references will be fine)
details about four specific events in which a person or business did
something that was legal at the time, but is now illegal (or just
“wrong”). Three places to start:

On 60 Minutes in October of 2000
(http://www.odwyerpr.com/archived_stories_2001/july/0718commentary_kevin_ge.htm),
Jack Welch said that between 1947 and 1977 GE had an EPA permit to
allow PCBs to drain into the Hudson River (when no one know they were
toxic). Now that PCBs have been found to be harmful, the EPA has fined
GE to cover the cost of clean up, even though what GE did was not
illegal when they did it.

Surely most of the asbestos-related injury cases must have involved
damage incurred before anyone knew it was harmful, but now companies
have had to pay enormous claims (even including those who are afraid
they
*might* become sick in the future).

Paint manufacturers used to use lead in paint, but most (I believe)
quit when they discovered it was harmful, even before it was made
illegal. Now, I’m certain, lawsuits must be pending.

I’m a financial planner and want to show retirement plan sponsors that
meeting the “letter of the law” (ERISA 404(c) – providing 5 investment
choices and a little investment advice) might someday be found to be
“wrong”. If that happens, it might put them at risk, even though what
they are doing is legal today. Obviously the punch line is that they
should hire me to give unbiased advice, both to help their employees
and to offer them some protection against a future re-interpretation
of the law.

A law that punishes someone for something he or she did when it was
not illegal is called an ex post facto (after the fact) law. The
Constitution prohibits them in criminal law, but U.S. courts have
allowed them in civil law (1).

Here are four examples along the lines of yours:

In 1969 and 1972, Congress passed laws making coal-mine operators
liable for the black-lung disease of miners, even if the miners had
retired or quit before the law went into effect. The Supreme Court
upheld the constitutionality of the law (2).

In the 1980s, the government sued Monsanto for dumping toxic waste,
suing the company under a law not in effect at the time of the
dumping. An appeals court upheld the penalty (3).

In 1969, the Supreme Court, basing its decision on a law not in effect
at the time of the eviction, ruled in favor of an public apartment
tenant who had challenged her eviction order. The local housing
authority had evicted her without giving a reason, a practice later
made against federal regulations (4).

In 1941, the Supreme Court said the company Owens-Illinois could be
held liable for an employee's injury under a worker's compensation law
not in effect when the worker was injured (5).

Note that none of the actions in the coal miners, Monsanto or
Owens-Illinois cases were ever "illegal." It was a question of
liability, handled in civil court. Even the case you mention, like the
Monsanto case, was not criminal, even though it involved the
government. It may not be "illegal" to leave a big pool of oil in your
store's walkway for two hours, but you may be liable if someone trips
and gets hurt (there is such a thing as criminal negligence, but I'm
not a lawyer, so I won't get into that). The public-housing case was
something in which no one was really punished, other than by lawyer's
fees. In American law, you can't go to jail for something that was not
illegal when you did it. You can, however, get sued for what you did.

I hope this answer meets your needs. If not, please request
clarification.

References

(1) Steve Selinger, "The Case Against Civil Ex Post Facto Laws," Cato
Institute
http://www.cato.org/pubs/journal/cj15n2-3-4.html

(2) 428 U.S. 1
http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&court=US&vol=428&invol=1

(3) U.S. v. Monsanto, 858 F. 2d 160
http://www.elr.info/litigation/vol19/19.20085.htm

(4) Thorpe v. Housing Authority, 393 U.S. 268
http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&court=us&vol=393&page=268

(5) Vandenbark v. Owens-Illinois, 311 U.S. 538
http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&court=us&vol=311&page=538

Search strategy:

ex post facto laws
://www.google.com/search?sourceid=navclient&q=ex+post+facto+laws

retroactive laws
://www.google.com/search?sourceid=navclient&q=retroactive+laws

Supreme Court decisions
://www.google.com/search?sourceid=navclient&q=Supreme+COurt+decisions

United States Monsanto retroactive 1988
://www.google.com/search?sourceid=navclient&q=United+States+Monsanto+retroactive+1988

I don’t know if I’m more pleased with your very thorough answer or
that Google has made this possible. I know I’d have spend hours on
this without your help. Thank you very much for taking the question
seriously and providing such a detailed response.