Of course I would allow a saving throw if you walked into the trap.
I've just finished catching up on the thread after the weekend, and this is one of many examples of you projecting bad faith play onto your interlocutor.
You can literally see in the post you quoted that I acknowledge you might have allowed the save. That doesn't actually address my point in anyway, because you wouldn't allow the check to detect the trap.
I noticed a couple of people have similarly distorted my example of the assassin in the alcove to turn it into some gotcha scenario where an evil viking hat DM is looking for an opportunity to stab a poor PC in the back if they don't say the magic words, rather than an example of a situation where a character can automatically see a previously completely hidden foe if they move into the right position. If they simply participate in play by describing what their character does or attempts to do, thereby giving the DM enough information to picture the scene and adjudicate their actions.
Because you are saying it is "an example of a situation where a character can automatically see a previously completely hidden foe if they move into the
right position." Well, if there is a "right position" then there is a "wrong position" and what happens to the PC if they move into the
wrong position? Would they not get stabbed by the assassin?
I read with a bit of frustration and bemusement the exchange where Celebrim gave examples of four garden walls, each with likely different difficulties to climb, but also potential hidden hazards, and
@GMforPowergamers seemed to breeze completely past the point of the player owning his character's decision about which wall to try. I disagree with how Celebrim uses the word Railroading (I subscribe to a purely pejorative definition), but he's completely correct that if I as the DM simply let the player roll athletics and then narrate him climbing over the easiest wall, I've taken control out of the hands of the player, and implicitly communicated that there were no hazards on any of the walls.
This example seemed poor to me for a few reasons.
1) You don't need to roll athletics to climb a wall. To quote the PHB "
Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re climbing, swimming, or crawling. You ignore this extra cost if you have a climbing speed and use it to climb, or a swimming speed and use it to swim. At the DM’s option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check." So, only one of those walls could even be argued to need any check to climb. The wall with ivy is trivially climbable. The other was a brick wall, so again, trivially climbable, and only the last wall was smooth, which may require a check. Of course, that also depends on if the wall is higher than 15 ft, because at that point most parties have at least one character who can jump up and grab the top of the wall, pulling themselves up without a check and lowering a rope, which would not require a check to climb. You may even be able to reach higher if you have an ally boosting you. Which, again, would probably not require any check.
2) It was never clear to me if these were all walls facing the same direction, or different directions. If they are three walls facing three directions, then the player needs to declare which way they are going, but that has nothing to do with the walls. So it seemed to me that they had three different walls, all facing the same direction, and all leading to the same place. In which case, the only reason it matters which wall is if one of them is trapped. Which just leads to you playing Three-card monte with the players. If there are hazards on one of the walls and not the others, and I'm forcing the players to guess which wall is dangerous, for no discernible reason other than to hit the players who guess wrong with a trap.