Does anyone remember in the book "Jacob Two-Two meets the Hooded Fang", the character of Master Fox who did toy shop sabotage: swapping jigsaw pieces from box to box? Also switching labels on tubes in chemistry sets.
Somewhat tangential, pondering over some maths here: is it true that when a puzzle says 1000 pieces, sometimes it's not exactly 1000, but actually something more mathematically convenient like 1024? (Sometimes the word "approx" might be in small print on the box.)
1000 is a slightly inconvenient number mathematically for a rectangular jigsaw with ordinarily shaped pieces, as it has (relatively) few factors:
1, 2, 4, 5, 8, 10, 20, 25, 40, 50, 100, 125, 200, 250, 500, 1000. (Have I missed any?)
The only "sensible" rectangle of exactly 1000 pieces would be a 40 by 25 jigsaw. Anything else would give a jigsaw which looks rather "widescreen", or even "letter box" shaped...