His executive toy

Do you remember Jean-Luc Picard, the captain of the Starship Enterprise from The Next Generation, having a large crystal on his desk which he often played with as if it was acting as a set of worry beads? This was apparently in over 70 episodes, yet most people, including hardcore Star Trek fans, claim to have never noticed it.

Sometimes a false memory can work in reverse, as in this case. Once the crystal is noticed, it becomes pretty obvious every time it crops up again, but some people swear it really never was there and it's somehow been inserted after they saw the shows.

The odd thing about this one is the very idea that Picard - famous for his "Make it so!" command - would need any kind of emotional prop at all. Yet, it's right there in all the clips and photos. At the moment...

Replica

Currently, the crystal is for sale at the Roddenberry Shop where it is indeed confirmed that it appeared in "no fewer than 78 episodes".

There's an observation from 2015 discussing whether the prop is the same as used in The Dark Crystal, which mentions it being seen a few times in seasons 1 & 2, and then much more frequently in season 3. It also comes as no surprise that Star Trek:TNG often deals with Mandela Effect related storylines, such as the Ship in a Bottle or Yesterday's Enterprise episodes.

Some people recall the crystal Picard held as having some kind of function sometimes, such as sensors, or even a weapon like a phaser. No one can remember the crystal playing any role in a story line, or even being mentioned explicitly even once, which might have gone a long way toward contributing even hard core fans to just blank it out, until being pointed out such as now.