This one might be a little more interesting.
You do this for long enough, time says you've seen all the variations and idiosyncrasies of a particular poster design. Well, a post on this forum lead me to Wolfgang's Kino poster site and there was a Spy 1 Sheet poster that had been sold, the image of which remained though.
It showcased an alternate type and colour for the tag line at the top of the poster. I was obviously aware of the fully red one, had relatively recently been introduced via Adam's site to the all blue one (and subsequently acquired), but this was red and blue in the same font as the all blue version.
A search in the Heritage and emp archives informed me that they had both never sold one...!
An email to Wolfgang got me the email address of the chap who bought it together with the information that he too, Wolfgang, had never noticed the difference. Emailing the buyer elicited no response. The poster itself was in fairly used and abused condition, but nonetheless...
Thinking I had come across something that would Never again be sold, I settled into a feeling of resignation. ...Until the very following week when Heritage popped up a mint and rolled version of the very same poster! Not sure anyone else noticed either (at least, of those who were in the market. A friend did also bring it to my attention) as it went for a reasonable price for a rolled 70's 1 Sheet.
Below the poster and the variations. The red and blue strip is blown up from the Heritage image, not something I took, hence the pixelated artefacts.