How tightly enforced are open-access embargoes? Many academic journals have copyright policies which forbid authors (possibly for a finite embargo time) to make their papers (either preprints, accepted manuscripts, or camera-ready versions) freely available (either on their personal websites or on repositories such as the arXiv). How tightly enforced are these policies? Are there known cases of publishers pursuing legal action against an author for posting copyrighted academic papers? Or would such cases normally be dealt with private requests to cease-and-desist? 

This answer seems to indicate such cases are rare, but there could be privately-dealt with cases that are not visible. Or is there a large body of public-repository-published papers that possibly / probably / demonstrably have been publicly posted in breach of a copyright policy?
One data point:

I am a mathematician, and I have never heard of this happening to anyone.

Many mathematicians post preprints on their website and to the arXiv, and it is somewhat common for mathematicians to also post scanned copies of the published journal versions of their older papers.

Ethical judgements may vary, but I am unaware of any authors who have suffered negative practical consequences for this, or even who have been asked to remove their papers.