Let’s say miner Moe mines a block taht is accepted by the network. Moe presumably receives some Bitcoins in his wallet for this. So let’s say Moe “spends” that Bitcoin by buying some anti-poking goggles from Curly, even though his newly mined block is at the top of the chain and has no blocks on top of it. Curly sends the goggles to Moe via FedEx. Then another miner Larry mines a longer block and that replaces the original one Moe mined. So now Larry has some Bitcoins. So does the transaction between Moe and Curly disappear and Curly is left without a pair of goggles and with no Bitcoin?