I love jigsaws, but DH doesn't, so he doesn't interfere. I quite often let him put in the last piece though, as he enjoys that and I have had all the fun part.
We knew someone who was a civil servant, with a responsible job involving access to confidential information, who applied to a charity shop (PDSA, I think) to check donated jigsaws for completeness. He had to have a disclosure check before they let him do it!
I have a 9,000 piece jigsaw of New York, which has only been done once as it needed a table tennis table and we don't have one (or room for one) any more
. The whole family was involved and it took ages.