No Contest - S1-E18
Revealing mistake: The photographs and news items relating to the Boston Barmaid Contest have been clumsily pasted onto a real newspaper. The paper upon which they were printed is cleaner and whiter than the off-white newsprint and the photographs have much greater contrast.
Revealing mistake: When Frasier comes into the chalet after skiing he doesn't show any of the visible effects after a hard day on the slopes. Meaning, his face isn't even flushed from the cold, his hair is perfectly combed after wearing a hat and there is not even a drop of snow on his boots.
Revealing mistake: When Rebecca is in the office and throws a drink on Sam, the pattern of wetness changes on Sams shirt.
Get Your Kicks on Route 666 - S10-E2
Revealing mistake: When the guys are stranded you can tell the background is just a backdrop as the scene is not perfectly flat - there are some wrinkles in it. (00:13:05)
The Girl in the Plastic Bubble (2) - S11-E7
Revealing mistake: When Frasier is on the window ledge the safety line preventing him from falling is visible in some shots.
Answer: Here is the source of the "WE WIN" photo: http://www.oldnycphotos.com/nyny587ac.html Brooklyn, of all places.