Yes, that is a very important loophole to bring up here. On one hand, parallel construction can be used in order to not compromise sensitive sources and method

Yes, that is a very important loophole to bring up here. On one hand, parallel construction can be used in order to not compromise sensitive sources and methods. If you found out about a spy ring because you compromised a spy's phone, you might not want that revealed, so a different path to the evidence is constructed to conceal that method. But it of course can be used in a nefarious manner to launder evidence gained on someone illegally. So maybe in the course of a nat'l security investigation the feds stumble conversations of a drug trafficker, then use parallel construction to create a paper trail that would make it seem like the knowledge about the drug trafficker was obtained through normal channels. That is absolutely an area that requires more rigorous controls and oversight, although I don't know how often FISA warrants have been exploited in that matter. Could be mostly federal law enforcement eavesdropping with stingrays.