This situation such as the one below came up in a hand last nite ( LC's Fun Tourney ), and as coincidence would have it, I saw a similar problem in the newspaper bridge column today.
You need all 6 tricks in a suit to succeed in 3NT. There are no transportation problems, but a common error is made. The 10 is led to be run.... but is covered by a stiff honor . Now there is no recovery. Down 1.
10 x
9 x x x K
A Q J x x x