There's a part early in Space Quest 1 where you ride a speeder into a desert town and the first NPC there offers to buy the speeder from you. The game straight up tells you that the speeder is useless now, so obviously you want to sell it in order to get the starting cash you need for the next set of puzzles in town.
BUT, if you say no and then come back to that screen later then the NPC will throw in a jetpack to the offer. You can get all the way to the end of the game without the jetpack, and then on the last screen you'll always die because you don't have it. There's no hint anywhere that this guy has a jetpack or that one even exists in the game, so if you missed the jetpack then you'd probably never figure out it was there all along.