Chandler asks Joey what Rachel's breast looks like, but he saw both of them in The One with the Boobies (1995).
When Joey is in the room with Rachel, the door behind him keeps switching from being open to closed.
When Ross is sitting in the hospital bed with Rachel and she is acting like she didn't hear his question, her hair keeps switching from in front of her to behind her.
Monica, who spent her entire childhood fantasizing about her wedding, doesn't recognize that the ring on Rachel's finger is her own grandmother's engagement ring which Ross had previously also given to Emily. All she has to say about it is, "It is bigger than mine!"
At the end of the episode, Ross noticed and said to Rachel that she's wearing the engagement ring. But earlier Phoebe showed him the ring, so he already knew.