Sometimes if my party wants to do something REALLY dumb that I think might be fun, I'll let them "quicksave" and run it through. Players get to attempt the impossible without fear, and I get to TPK without guilt =D
You could feasibly fit it into the actual lore and make it "balanced" in a sense. The party finds a relic with the power to rewind time by a few moments. Becomes inert for a few days/play sessions after use.
Then there is actually the potential for complete failure (the relic holder dies instantly before being able to activate it), but they still get a sense of safety that propels them into stupid dangerous stuff all the time.
Hourglass of Lost Chances
Wondrous item, legendary (requires attunement)
This magical hourglass, crafted from a material more resilient than steel yet as transparent as glass, contains sand that remains curiously still and does not flow.
While you have the Hourglass on your person, you can utter its command word as an action to activate it. Upon activation, the sand begins to flow from one bulb to the other, a process that continues unabated even if you turn the Hourglass upside down.
While you have the active Hourglass on your person, you can utter its command word again as an action. Doing so reverts the timeline to the moment you activated it. Every event, including death, is undone, but all creatures across the multiverse retain their memories of what transpired during that timeframe. However, any creature that was not within a 1-mile radius of you at any moment while the Hourglass was active experiences this as a sense of déjà vu.
The Hourglass becomes inactive 10 minutes after activation or immediately after you use it to revert the timeline. Once deactivated, it cannot be activated again for the next 7 days.
In one of the games with my group we screwed up getting to a heist so badly that 2 of us ended up downed and the other 3 were facing prison. That's when our bard decided to use Wish to try to jump us all to a timeline where our initial plan had worked. The DM let him do it and go back to playing the heist itself.
The DM also used it as an opportunity to reveal some lore, and as we transition to the new timeline we got to look in on some others where the BBEG was winning.
This was 2018ish and we still talk about it. Save scumming can be a lot of fun in the right context.
