The flash forward scenes only take place 10 years in the future making Tess 19 and Annie about 16-17. Tess however is already a social worker and Annie is older than she should be as well.
Often Randall is referred to as Kevin's "little brother", however, it is later revealed that Randall was born the day before before Kevin and Kate. But the decision to take him in was after Kate and Kevin's birth.
Kevin and Kate are often referred to as twins; however, although the third baby died, technically they are triplets. However, as said, 3rd baby died during childbirth, and the 2 were inseparable.
During flashbacks to the 70s and 80s that are in grocery stores, the products on the shelves are often in modern packaging. Only products that no longer exist are shown correctly. In one scene Jack is next to single serving plastic bowls of Kellogg's cereal. These didn't exist in that time period. Only single serving boxes were available.
Mackenzie Hancsickak who plays the younger version of Kate has darker eyes than Hannah Zeile and Chrissy Metz who play older version of Kate. Both Zeile and Metz have blue eyes.
The slang in flashbacks is often not yet in use. In one scene set in the late 70s early 80s Rebecca answers an observation that Jack makes by saying "Right?" Which developed alongside "I know, right?" Both of which only came into use in the 2000s.