Kilus said:
if a game designer has a guard that says one thing ... but they want the player to do the opposite ... then that needs to be communicated to the player.
While true, this isn't necessarily a DIRECT thing.
For instance, as with your Portal example, an obstacle can be placed and it's due to "prior knowledge" that the player knows they can get around said obstacle. There wasn't any immediate communication, and if there was it would likely ruin the experience. Again, think your Portal example: if at EVERY puzzle room the game gave a hint to let you know what to do, that'd ruin everything, wouldn't it? Instead, it's the experience of the player having ALREADY gone through an exercise that "communicates" to them the message "I can get around this".
On the other hand, you can assume that "prior knowledge" won't come into play very often in playtesting, because these testers aren't playing a complete version of the game, having gone through character creation, tutorials, and the whole gambit of gameplay up until this obstacle. But there are still other ways to communicate the intention, not small part of it is the acting of the NPCs. Even in non-dialog interactions, the way the sentences are conveyed (stressing words, or describing behavior) can communicate simple notions to players like "He's saying I 'cannot' go upstairs, but I'm hearing that he doesn't 'want' me to go upstairs". It's not a hard thing to do, sometimes it's just not done. I know that there's MANY instances of NPCs in New Vegas telling me "don't even bother, it won't do anything", and every single time I get the message "you can, but I suggest you don't", and most times I opt to try.
There are a myriad of ways to make sure players "don't get lost", and they DON'T require holding their hand. Even in a literal sense. Some games come with no built-in maps, and they force the players to form their own sense of direction to travel without getting lost. It doesn't hurt players to encourage them to utilize more enginuity and creativity, as long as you let them know what they're CAPABLE of. That's all it takes, clearly explaining the "rules of your [game] world".