But, more importantly -- if you are going to rely on saving throws, then the dice are telling you something about how the situation played out.
So are the rules. And if there's a CON save for that poison, that's something that should come
after establishing that the poison was touched.
versus: The chest has been smeared with an almost imperceptible, deadly contact poison. Any creature touching it must make a saving throw versus poison or be killed instantly.
In the second example, i would suggest the result of the saving throw could potentially inform us about the presence and efficacy of gloves. Are the worn from battle and adventuring -- that makes sense if they failed their save despite wearing gloves.
Didn't you tag this thread "5E", not 1E or 2E? If so,
what on earth is a "saving throw vs. poison"? I'm guessing this means a CON save but...
I could see this case being made for the approach to saves in 1E/2E. It's not perfect, but I kinda see it because those saves so extremely abstracted.
For 3E and onwards? Nah. I can't see it at all. I could see maybe some kind of separate, novel saving throw or check added in for what you're describing, basically a "luck save", before we move on to the CON save. This is how a lot of saves work in wargames.
As a DM, I'm never going to say a player did a thing they didn't say they did. I might like, get them to be very detailed, and walk themselves straight into the problem, but, I don't think there's any need to go further.
I mean, players are idiots already. Even smart players are idiots. No-one is going to convince me otherwise when 34 years of experience and every single podcast, YouTube, and story about players (going well back into the '80s) supports this. You don't need to gotcha them. You don't need to revise what is going on contrary to what they're saying. You didn't get them that time? They'll get
themselves in 10 minutes dude.
This is what I don't get about a lot of the people rushing to get in the gotcha here. Why? Who does it really help? You're going to get them real good sooner or later. Okay they wore gloves this time, noted, that's why use a poison needle trap, not contact poison.
(As a total aside, long-lasting contact poison is just a really dumb and implausible idea generally. Virtually every kind of slimy medium which could hold a poison is going to either dry up, slide off, evaporate, or all of the above, within like, hours to days. And if you want the trap to be magic, just say it's magic, don't try and sell me on a non-magical goo that does none of those things! I basically never use it because it's so implausible. I don't like traps that make no sense at all, and that couldn't last. Any trap that's finicky or fancy like this would need maintenance. There'd probably need to be a kobold round every 24 hours to re-paint the poison on it, and there'd be signs of that and so on. I always think every part of a trap's lifecycle through.)