For example, during a patent case I was working on, we found a memo from the opponent's attorney saying that the Patent may be invalid because they filed it too late. A disgruntled former employee of the company had a carbon copy of the memo. For some reason, it was not on the opponent's privilege list - they claimed not to have a copy in their files! We were aware of the document and it could have torpedoed their case as it was an admission that the patent was filed too late. But, the opponent claimed the memo was protected by attorney-client privilege, and thus was inadmissible as evidence. We had a hearing, and the judge reviewed the memo and agreed with our opponent. If we were to prove the patent was invalid, we would have to do so through other means. This did not mean the memo was kept secret form us or that we instantly forgot that it existed. Of course, we could not disclose the contents to others.
But the idea that you can use a lawyer as a means of shielding your activities from public view or from the view of the police or the courts, is flawed. You cannot avoid prosecution from crimes simply by claiming attorney-client privilege, if you and your attorney are in fact, partners in crime.
But again, a lot of people misunderstand this.