Are there any dangerous consequences when allowing spending inspiration on rerolls?

Question

In D&D 5e Inspiration can be spent to create an advantage. My group introduced a house rule:

If you fail a d20 roll you could've used Inspiration on, but before DM narrates the effects of this fail, you may spend inspiration to reroll. It doesn't count as advantage, and if you rolled with disadvantage you also reroll with disadvantage. You must use the new result.

On one hand, it can save the party in a tight spot and prevent one bad roll from ruining the story. On the other hand, it doesn't trigger sneak attack or any other effects that happens when there's an advantage, or no disadvantage.

Are there any unintended, surprising consequences of this rule? Anything I should be aware of? Crazy combos or edge cases that could derail a game? I know it makes Inspiration slightly more powerful and much more versatile, and I'm okay with it. I just don't want exploits.

I'm the DM now, but I was a player in the Avernus campaign where we also used this option, and it seemed safe enough. In both campaigns inspiration is fairly common and each player can expect to be able to use it every hour or two.

Answer 1

No, there are not dangerous consequences

In playing since 2014, and DMing since a few years after (against my will at first) I have found that Inspiration does exactly what it is intended to do: now and again you can overcome a bad roll.

We have done it both of two ways.

1. Roll with advantage when you make an attempt, a saving throw or an attack. This works best with players who pay attention to detail so that they call out the advantage roll in advance. Any roll with advantage tends to have a decent result. (Usually, not always).

2. Roll again after a poor roll. This is far easier to implement.

If you fail a d20 roll you could've used Inspiration on, but before DM narrates the effects of this fail, you may spend inspiration to reroll. It doesn't count as advantage, and if you rolled with disadvantage you also reroll with disadvantage. You must use the new result.

That works a lot like point number 2. Your house rule is fine.

How often you award Inspiration becomes the bit that you control as a DM. If it is rewarded now and again, the math factor gets lost in the noise. If you award it a lot, which I have seen in play, it still does not have an overbearing result.

If each player has one Inspiration point at the beginning of each session (many groups I have played with have done this) we discovered that about half of them never get used because players forget to do it.

Option 3: I can pass my Inspiration to another player

From the DMG entry on Inspiration.

Players and Inspiration. Remember that a player with Inspiration can award it to another player. Some groups even like to treat Inspiration as a group resource, deciding collectively when to spend it on a roll. It’s best to let players award their Inspiration as they see fit, but feel free to talk to them about following certain guidelines, particularly if you’re trying to reinforce conventions of a certain genre.

We have found in the past six years that this is a great way to boost both teamwork and team morale. I strongly suggest that you adapt this convention. Both @V2Blast and @MikeQ (who are veterans of this site) are in my Saltmarsh campaign. We have adopted the DMG suggested option of passing Inspiration from one player to another, both in and out of combat.
It works.
It is a good idea.

Award Inspiration and use it.

My experience over just under 10 years is that your fears are groundless. The occasional reversal of a bad roll usually creates a boost of positive energy at the table on both sides of the screen.

Enjoy.

Answer 2

It's Quite Safe

The proposed option in your house rule is a similar (but less powerful) version of the 1st feature of the Lucky feat:

You have 3 luck points. Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined. You choose which of the d20s is used for the attack roll, ability check, or saving throw.

The houserule described inherits disadvantage and can only be used once per inspiration, which at most tables is fewer than 3 times per long rest.

The Lucky feat not only does not inherit disadvantage but instead grants superadvantage, as unlike your houserule Lucky lets you choose which roll to use. Some might consider this a crazy combo or exploit -- but you've worded your houserule not to include this.

Additionally, the Lucky feat even lets you influence other creatures' attack rolls.

In short, seeing as your proposed inspiration houserule is weaker than half of a PHB feat, I'd say it's quite safe.

Answer 3

I've never had any problems with it

I've played in many campaigns with a very similar house rule, and even DMed a few with it. It has never caused any unintended consequences, which is to be expected given it's similarity to other features, feats and spells already in the game (As pointed out by other posters).

One aspect that jumps out to me as unique is that it allows a reroll after the result of a roll has been called. Aside from the spell Silvery Barbs, I'm not aware of any other mechanics that allow something like this. It could end up feeling a bit cheesy, as a sort of undo button, but it may well be that this fits well into the story you're trying to tell as a DM.

That being said, in the campaigns I have played in, inspiration has been handed out very sparingly. Generally, it has only been given out for extraordinarily good play, meaning that it might only be given out once every 10 sessions, if that. The house rule could easily lead to problems with balancing if you are prone to handing out inspiration constantly, and it could turn into a situation where it is near impossible for players to fail. Whether that's a problem or not depends on the type of story you are trying to tell.

At the end of the day it depends a great deal on your players, and you should examine how it is used by the players at your table to see if it is having its intended effect or if it is being abused.